Can you move him into your bedroom? He may sometimes get lonely and bored alone in the basement. If you move him into your bedroom, he will bond more and may even sleep on the bed with you. Also begin introducing your other cat at least by scent swapping (putting something of his with new kitty and visa versa). Are they sniffing at each other through the bottom of the door? My experience has been that feral cats adapt very quickly to other cats. My last feral girl kitten was bonded with my four resident cats in a few weeks, while it took four months for me to be able to touch her. Also leave out catnip toys for him. He will likely play with them at night. If you can take a picture of his genital area, someone here can probably tell you whether or not he has been neutered. I think that once he has the run of the house and companionship with your other cat, he will likely be happy to remain indoors. If he were still crying, or caterwauling and scaling the walls, I would suggest to release him, but it sounds like he is making himself at home with you. It sounds like you've been making great progress!