I'm also against testing, which is why P and S have never been tested. They were feral/semi-feral before I took them on, so have most probably been in contact with the virus, as have many cats.
A lot of cats have a natural immunity to FLV/FIV, catch the virus, then throw it off. These would nevertheless test positive to the anitbody test, as the antibodies developed in throwing off the virus (conferring lifelong immunity) would still be in their body. Many cats carry the virus, but never develop the actual illness, instead they die from kidney disease, old age, RTA, or any of the other things which kills cats. Of those who carry the virus, only a small proportion go on to develop illnesses related to it.
One thing I discovered years ago when Minnie was ill was that very few vets understand the pathology of FLV or FIV and I was having constant arguments with those who wanted Minnie pts because she had an illness and was FLV positive. My argument was that the existence of the virus was coincidental. It was, she lived a healthy life for another 4years.
When I found a vet who knew more that I did, I stuck with him. He didn't advise pts, but treated the infection, which she overcame.
If you do have Seb tested, make sure you have the more expensive VIRUS test, not the ANTIBODY test. Many cats who have thrown the virus off will test positive for the antibody test when they do not carry the virus.
I wouldn't have him tested, but would introduce him into your household.
If your child was HIV positive, would you have him/her euthanised? Or isolated? Surely you'd just avoid body fluid contact with other children. The infection process of FIV/FLV are very similar to HIV, except that cats have an immunity to their diseases, humans have no immunity to HIV. (By the way, there is no cat to human transmission.)
Whatever you decide, I hope all goes well.