We foster a lot and the fear of a new cat is typical behavior among cats already in the home. Most cats aren't easy going with new kitties coming in. Patience is required; they will work it out, but they need time. Continue supervised visits for a while, lengthen the time of each visit and give them reassurance.

Speaking as a former animal rescue volunteer, I have to tell you that as soon as you start feeding this cat, you've signed an unspoken contract with it. It sounds as though, from his behavior and reactions to you, he is definitely a stray. You have two choices. If you stop feeding him, he will eventually find someplace else to go for food. If you continue to feed, you'll need to trap him (call your local police/animal control for loan of a trap), find a vet who can handle a feral (local rescues can be of help to you there. They may also be able to send one of their volunteers out to help you) and follow their direction. You can vet, neuter/spay this cat and return him to the outside. Do not let him in amongst your cats. He could prove to be aggressive and also be carrying one of those nasty kitty viruses.