He needs to see the vet as soon as possible. I had an older kitty (she was about 14) who did this because her stools were so hard they hurt when she pooped. Your guy is probably trying to ease the pain of the poops.
Your kitty, at 16, is very set in his ways and is probably upset about the two new kittens. He may have an upset tummy from a virus or from stress, or another health problem, that is causing his diarrhea. It is important to take him to see a vet as soon as possible. In the meantime, give him extra love and try some wet food. I hope he gets better soon!
Cats do this when they're trying to get rid of something they can't poop out or their anal glands need expressing. I have one with anal glands, and she scoots when they are too full. Sounds like a vet visit would be a good idea.
I agree that your kitty needs to visit a vet as soon as possible. Dragging her butt on the carpet could also be a sign that she needs her anal glands expressed.
If you're not squeamish and it ends up being full anal glands, you should ask if your vet can show you how to express the anal glands yourself. With some cats, it ends up being chronic problem and they need them expressed every couple months. But it could also be constipation or an anal gland infection.
This may be the case but for now vet assist is essential. Also when he-she goes #2 look at it, manually, I know it's gross, to see if the center of it is dried out and or actually hollowed out.
My cats were presenting this and I introduced wet food intermittently to solve this dilemma.
In case it wasn't clear, I wasn't suggesting that you try to deal with his butt yourself, just that if it's an anal gland issue your vet can teach you how to manage it at home so you don't have to go back so often. Anal gland issues can be serious and left untreated too long can lead to rupture. Worms can be another possibility, especially with the squirts. And if it's only a small amount of diarrhea coming out, it's still possible he's constipated (sometimes a little can leak around the hard piece clogging everything up).