It's wonderful that you already had all of that done, so when you do take him in again, there's no need to worry about all of that. Maybe just dosed him with some Frontline Plus. It may be that he comes in for short visits that gradually get longer. Some people have socialized outdoor cats to become indoor cats that way. Are you able to pet or handle him at all? Your cats are becoming familiar with his scent, and even if he rubs against your legs, they will become more familiar with him. It's wonderful that you are helping him.