You know, from that description of him at the vet, he's sounding way more tame, than feral. I mean, just regular house cats can freak out more than that at the vet. For that matter, I have a harder time getting my Ruby into her carrier for a vet visit, than it sounds like you did with Andy.
Have you considered getting a large dog crate to keep him in to introduce him to your other cats. Since he's been neutered and vet checked, (vaccinations too, right?) there'd be no health danger between him and your cats. So you could put him in there, with food, water and a litter box, and keep it covered with a blanket, and he'd feel safe, and he and your other cats could get close without actually more than nose or paw touching. I think that's how Jcatbird sometimes has introduced her ferals to her other cats.
@ladytimedramon posted this link in her own thread that might be helpful: Introducing a new cat - the crate method - Adopt-a-Pet.com Blog
Yes, I actually have a very large dog kennel, that could fit a great dane, I could easily fit a litter pan, and "nest box" in it, and cover at least part of it with blanket, or tarp, I may do that, I don't want him to feel "trapped" but if I put a large nest box in there, I think he would feel safe in it.
Yes , he definitely is not feral anymore, at least not with me, but it took probably a good 2 or 3 months before I could touch him, he would show himself, but not let me close, and hissed , when eventually I could get within arms reach, then after several more months we became friends, he did once jump on my leg , and scratch me, so I had to start wearing coveralls, it was like he was stimulated though, I had been scratching his rump area, and stood up to leave and he went after me, now, and before I took him to new home I can put my face right up to his, and pet him anywhere with no fear of attack, I was crying morning he showed back up, and he was high up on a pallet of bags of salt, so kinda level with my head, and he just reached out to touch me with a paw when I stopped rubbing him, and his face was an inch from mine.