Becky, I'm not sure what advice to give about the biting. We don't allow our cats to bite, and discouraged it when they were kittens by giving them a puff of air in the face when they would bite. They quickly learned that wasn't acceptible behavior.
Tuxedo just recently started giving Gary what can only be described as "love bites." He puts his mouth on Gary's arm, quickly and lightly while looking up at him. Gary then leans over, and Tuxedo bumps him on the head. So we don't discourage this, because it's his way of communicating to Gary, lean over, I want to bump your head! Tuxedo is the only one that does this.
It does mean FOR SURE that Lil Buddy is feeling better.
When Tuxedo was near death because he was so anemic, he would let anybody do anything to him. Cats can be very loving when they're sick - or at least seem very loving. Sometimes it's because they like the attention when they're sick - other times it's because they don't feel well enough to do anything about it. Tuxedo didn't need a tranquilizer for a six hour blood transfusion - and this is a cat that sounds like a German Shepherd or Cougar when anyone approaches him with a needle. He's bitten two vet techs and Gary in the past (in the hospital) - one of techs required stitches. The first sign that he was feeling better after a few weeks was that he'd growl and hiss when getting his shot - not just lie there and let someone give it to him. Now I hold his head and shoulders under a towel while the vet gives him the injection. And when we go for weekly blood work, if you were someone just sitting in reception waiting for your pet's appointment to see the vet, you would swear there was a wild bobcat or cougar back in the operating staging area. I've never heard another cat make a noise like that. So I take this definitely one way - he's feeling better, and he's starting to let you know what's OK and not and when - or that he loves you, lol!
If it keeps up and it's not clear that it's love "nibbles" or a soft polite request of "stop that," then no matter how much you don't want to, I'd start correcting the behavior by giving a short sharp puff of air in the face when he does it. Personally, if I figured out it was his way of saying "no," I'd still correct the behavior. He'll figure out an alternate method of letting you know. Our cats all hold out one paw and put it on top of our hand. It's like they're saying "put your hand down, I don't want anymore brushes (or pets)."
I know how much you want him just to love you, but believe me, you will both be happier in the long run if you understand each other and communicate with each other what is OK and what isn't. It is the hardest thing in the world to say "no" to a kitty you're trying so hard to have love you, but you have to lay the ground rules or you'll end up miserable in the future with a cat that bites you - whether he has teeth or not, it's not OK.
If he does it when you're putting food down, perhaps its him being hungry?
If he does it when you're petting or brushing him, I'd say it was overstimulation. Tuxedo just puts his paw out now - puts it on top of our hand and we stop, though he used to bite (not hard). He often wants to be brushed again within a few seconds - but he can't take "lots" of love at one time. A few brush strokes at a time, pause for a few, a few more brush strokes, pause for a few - and that's progress. He's lived inside for a year now, and at first he couldn't take any pets or brushes, but he would headbump Gary if he leaned down next to him. And until just a month or so ago, the most he ever wanted was just a few brush strokes and that's it. He won't sit on laps, rarely likes to be picked up - but automatically kneads when placed on a sheepskin (which he would jump off of at first!), and he purrs when you just stand next to him. But it took a year to get there. So he lets us know in all of his odd little ways that he loves us and likes being here. But he doesn't really like the other four cats, mostly avoids them, hits them on the head when they get too close, and the only one he really tolerates and plays with is little Flowerbelle, though he won't get up and move if Shelly or Lazlo come near.
Since he didn't seem to mind when you continued petting him, I'm not sure how to take this either. Next time he does it, I'd stop whatever I was doing, whether it's brushing or petting and then see what happens. When Tuxedo first came inside, we would wait for him to come over to us and ask for pets before we'd try to brush or pet him. I know the situation is different for Lil Buddy, which is why I think the next time he does it I'd just stop and see what happens. Maybe put him back in the cage pretty much right away. If he doesn't like being in the cage, he'll learn to stop biting.
Also, I just wanted to add, you said
|I swear, this guy has his ups and downs.
This is always the case when socializing ferals - especially when they come in sick and are on the mend! As I pointed out they're often much friendlier when sick, and when feeling better they'll start exerting their likes and dislikes. But it's also usual to have a kind of "Three steps forward one step back" (sometimes even "one step forward and three steps back"!) in the process of socialization. Just do your best to figure out what he's trying to communicate. We'll try to help, and most importantly, don't get discouraged! It took Spooky three months to stop being aggressive with the other cats - it took her over a year to stop bolting into hiding (at first for hours) at the door opening or closing or any loud noise. She's been an inside kitty for a year and a half now, and she's become a lap cat. Tuxedo's been inside for a year, and he can barely stand being brushed or petted, yet he lets us know he likes it here. Lazlo doesn't mind pets - but LOVES being brushed on my lap in the bathroom. Nowhere else. In fact, he meows in front of the bathroom door when he wants brushes. Each one is completely an individual, and they all took completely different amounts of time to get used to us, each other, and our home. Shelly, the alpha, owned us and our home from day one. He explored the place from top to bottom and it's ALL his.
We live in an RV and have gone to great lengths to make sure there's enough room for all of them and us - and Tuxedo still hasn't explored several areas of our home! He was rooting around up at the front of the RV for the first time ever just a few weeks ago. So go figure.
Most importantly, you're doing GREAT! And remember - sometimes the best thing to do is slow things down. In the process of socializing this gang, there were times when it seemed like one or the other of them hated us. That's when we'd just ignore that cat, except for play-time with a wand toy for a few mintues a day. After a few days, they'd independently come over to us - either to rub up against our legs, or looking for a pet or treat or something. I know Lil Buddy isn't out and about yet, but the advice here would be maybe to give him a little less attention, and just be in the room with him with his cage door open. Read a book or something, and just "be" with him, not necessarily interacting with him. After a few days of this, maybe he'll approach you? With cats, especially ferals, sometimes "less is more."