As we had a similar situation recently, I understand how you feel.

You get so attached so quickly

We had two different results Positive with sweet Caramal & Negative with young Bramble.

We had Caramel PTS as he had Aids & mouth problems (very likely cancer). We wouldn't have released him again but if he JUST had the Aids problem only, we would have taken him to the Cat Shelter.

People adopt cats with Aids or Leuk. as long as they stay inside and have no contact with healthy cats, they can lead happy lives.

Here is my advice to you.

First don't panic, just wait for the results. Our vets does it while you wait (one hour wait for results)

If he is negative.. Great. Enjoy your new cat.

If he is positive... Don't release him, he'll infect other outside cats and add to the local feral problem. Ring around the cat shelters and ask if anyone would be willing to rehome him.

If you can't find a shelter to take care of him, you have two hard options, risk housing Oatmeal in your spare room and try to rehome him locally or have him PTS.

The chances are that Oatmeal will have negative results, as he was once a family pet. He should have had all his shots while he was a pet. The most recent thought is that vacaines can last for years, so hopefully he'll still be covered.

Thinking of you both.

PS. Keep us posted asap