Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned about Blix picking up Feline Leukemia from stray and feral cats that enter your yard.
Cats are most susceptible to picking up Feline Leukemia when they are young, especially if they have never been vaccinated. Adult cats that are vaccinated rarely pick up the virus even when they are exposed, but it can happen.
Also Feline Leukemia is passed via viral particles in the saliva, feces and urine. Mutual grooming, sharing food
or water bowls, and/or sharing a litter box are all possible ways to pick up the infection. So even if Blix doesn't purposely interact with these kitties if they are using the same areas to eliminate, and eating and drinking out of the same bowls it is possible that he could pick up the infection. The more he is exposed the more likely he is to get infected.
Odds are if he is fully vaccinated he won't get sick, but his repeated possible exposure to the virus is worrisome.
If he has never been vaccinated then I would have him tested and if he is negative I would vaccinate him. Likewise it it has been a long time since he has been vaccinated he should have a booster. He should not be exposed to these kitties again until 2-4 weeks after his final vaccine or booster.
As an aside my other concern is FIV, Feline Immunodeficiency Virus. Although he isn't interested in these kitties they may be interested in him. FIV is passed via blood, so fighting and sexual contact are the usual ways of transferring the virus. If he is attacked and bitten by a feral cat he could get sick.
The same could be true for Rabies.
If he is not vaccinated for Rabies that should be done immediately.
You may also want to build a caged in outdoor area just for him to decrease possible contact between Blix and feral/stray kitties.
Please let me know if you have any further questions.