I just stopped fostering feral cats. after i ended up with two i decided it best to stop my boy Cookie came out of a feral cat colony he was about 4 months old he is still really stunted so if your little boy dose not grow too fast don't worry as long as the vet says hes fine.
I would get some chicken jerky (the catswell company makes a good cat treat
Amazon.com: Catswell VitaKitty Chicken Breast Cat... )
when he starts to hiss back up and offer him a piece then when he takes it walk away
I always let the cat come to me going after them only ends up with bad scratches
try to interact several times a day as often as he will let you when he walks away board or upset let him he is more likely to come back