He sounds like a wonderful cat. Thank you for taking him in. But, you don't have to separate him from the others because of the FIV. Unfortunately, many vets aren't up to date on their information about FIV. It's not easily spread between cats and can't be spread casually like through litter boxes, water and food bowls, or when snuggling and playing. The virus is primarily spread through blood tranfusions or serious, penetrating bite wounds. Wounds of this kind are extremely rare, except in free-roaming, uneutered tomcats. So, as long as everyone gets along, this sweet boy should be just fine with your others.
I know many people who have cats who are FIV positive and negative living together and it's not issue. I've fostered two FIV+ cats myself in the past year, both of whom had contact with my own four, who are negative, and it wasn't a problem at all. If I was in the position to add another permanent cat, I wouldn't hesitate to adopt one with FIV and make him or her a fully integrated member of the household.
I hope you decide to keep this sweet boy. He sounds like a wonderful cat.
If you need more info about FIV, feel free to PM, and I'll send you some links.