Step Cat

We have a few stray cats that venture into our back yard from time to time. One of them is a regular and he comes by most days. He was a big bushy chap when he first started paying us a visit and we called him Bushy La La and the name stuck. He was a little skittish at first. I'd leave food down but he wouldn't come near it until I backed away or went inside the house. Now he has his own bowl, let's me pet him, and is quite the chatty fella. Some days he'll just sit by the patio door and look at me until I feel guilty. I know he wants in, and I would love to give him a home. The problem is we have two indoor cats already. Even if we did get him tested, neutered, and vaccinated, that cat has seen some shit. Our cats are not street wise, I worry there would be some kind of late night rumble they would not win.