Talked to the vet

Well, my SO called the vet to ask about this whole Feline Leukemia Virus issue.

And the vet sounded very confident, we shouldn't need to worry about it.
We don't even have to come in and get the younger cat checked out, unless of course he start getting symptoms of anything, which he most likely won't.
So I've calmed down and can enjoy watching the cat doing whatever he's doing, without fearing that he'll get really sick.

Phew.