URI - How long to quarantine?

I have a kitten who is recovering from a nasty URI.  The past few days he has drastically improved - his eyes are clear (still medicating), he is eating on his own and very well, and he is playing with toys.

Now that he is feeling better, his patience at being confined has disappeared.  There are other cats in the house, but he so far has no contact with them.  He is in a bathroom with the door shut.  That door leads into a bedroom with the door shut - so the bedroom serves sort of as an air lock.. I have a foster kitten who is too young to have been vaccinated and a (very) senior cat of my own who I have been trying to keep safe from whatever this poor kitten has.

He was at the vet recently, but I was so worried at the time about getting his eyes under control ( I was worried that he was developing adhesions, but this was thankfully not the case) and about making sure I had the updated meds and scheduling down that I didn't think to ask how long it would be before he would no longer be a threat to the other cats.  I spend as much time with him as I can.  He is playful and very affectionate but - let's face it.  Next to another kitten or cat I am chopped liver and he would much rather be out playing.

He has no more discharge from his eyes or nose and the wheezing has stopped.  His eyes are clear in the mornings - I'm no longer needing to remove a lot of gunk to get them open and medicated and the diarrhea is gone.  He's snuggled up with a stuffed moose and a lamb, which I hope is giving him some comfort but he is starting to spend a lot of time waving his paws around under the door and calling to get out.

Snce no one has replied yet, did you CALL your Vet and ask them?  I honestly don't know the answer, and every place I look up simply says to quarantine them or isolate them   In my gut I feel it's probably ok to let him out at this point, but I would hate to give you bad advise, and I've never actually experienced this myself to know for certain.

He's a foster (who I will almost certainly end up keeping), so the vet will only speak with the shelter about him.  I asked at the shelter today and their advice was to wait another week, so he will just be stuck with me as his playmate until then.  I guess nobody really knows how long he is contagious without knowing what caused the URI as there can be a few different viruses or infections behind it.  I put a new scratching post and a few cardboard boxes in with him today along with some larger sheets of butcher paper and this seems to be keeping him amused enough for the time being.  It is nice to finally seem him starting to bounce around like a kitten at least.

Ah yes, boxes.  The best toy in the world

That's why I can never foster...I'd end up adopting them all   Never saw I cat I didn't fall in love with. 

Kittens OFTEN get the herpes virus which causes the URI symptoms and the eye discharge. Get some L-Lysine treats from Amazon and keep them on hand for flareups. You can get them with immune system builders too, which wouldn't hurt. If your kitten was on an antibiotic, after 2-3 days he should be able to mingle. If your vet thought it was the herpes virus and didn't use a antibiotic, he should be fine now. I have one cat who gets the virus, he is never separated and the rest never come down with it. Your senior most likely has been exposed to most viruses by now and should have immunity, call your  vet to find out. I would call anyway and ask if he is safe or if they recommend further isolation. It's always scary when you don't know what he could have picked up, but his improvements are encouraging that it is something simple. Good luck!