In the house anxiety

Several years ago I took in a stray female cat who has worked out amazingly well, but she was lonely in the house all day while I was at work.  So when she brought home a feral friend (who I suspect might be her son/brother) I started feeding him, although I didn't hold out too much hope of taming him because he was completely wild at the time.  After 3 years, and endless patience, he's made it to what I'd call skittish friendly.  He's been neutered, gets his shots every year, comes inside for breakfast and dinner most days, and has his own little house on the porch.  They hang out together all the time in the good weather, and he will spend time very contentedly with me in the garden.  Inside the house he's ok with me picking him up for short periods, and will allow me to pet him almost endlessly.  He's never been forced to stay in the house, nothing bad has ever happened inside, and he's always allowed to leave on demand.  I have no expectation that he'll ever choose to live inside, but he's always anxious, can't settle long enough to nap or bathe, and when not playing with his toys or being patted will wander around howling for no apparent reason.  Sometimes he actually wants to leave, but more often he just wants to look outside.  Ideally he'd like me to leave the door open so he can sit in the doorway--not an option in sub-zero weather.  What I'm wondering is if anyone has ever successfully gotten a feral cat past this kind of anxiety?  And how?

Feliway......cat pheromones, look it up

Feliway will help him settle down, although it won't change his personality.

He may just be a jumpy cat.  Who knows what he might have faced outside, so he may have good reason to be nervous.

Is there a chance you can build an enclosure (also called a "catio")?  It is an area either screened in or covered with wire so the cats are outside but unable to wander too far.  It might give him the idea of freedom without the dangers.  You can make it accessible via a cat door, so your heating bill doesn't go through the roof!

Quite tremendous progress you've made with him! If nothing else, cats will teach us patience. Sounds like your guy wants the indoor comforts but the security of knowing he can escape if necessary. The howling may have nothing to do with wanting out. Some just like the sound of their own voice (I know a lot of people like that, too ) or maybe he's bored.

I've got one boy that was a wary, savvy, mistrustful stray when he first showed up in June 2011. He is a very smart boy that took me 10 months (a record which still stands) to catch for neutering! Long story short, because of a UT blockage I brought the big guy inside the house in Jan. 2013 and he's been in ever since. The first month he spent confined to one room both because he was ill and because there are 6 other cats in here, but thereafter he had free range of the house. None of the house cats, including him, go outside, but they do have a catio. He slept under the couch for the first 4 months, and that is still his safe place. He still gets anxious and runs from me sometimes if he feels I am approaching him too fast or he feels trapped in some way. And eating is still a problem with him, he won't eat with any humans anywhere near him. But looking back on it, I realize how far he's come and recognize that every day he gets a little more comfortable. He's never made an effort to escape out a door.