4yr old pitbull/mastiff rescue dog?

I rescued Zeus a 4yr old pitbull/mastiff. He is a great loving, caring dog with the exception of two issues. I have one son that lives with me full time (16yrs old) and a son (15yrs) who is only with me part time. Anyways Zeus for some unknown reason doesnt appear to like my visiting son very much. I introduced them like i would if he was a stranger given he doesnt live with me full time. My son hasnt done anything to the dog. Well the dog lunged at my son and left a scratch on his hip. Then we tried the re introduction thing, the dog was friendly and even started to kiss him on the face only to lunge at him again 3hrs later. I truly dont understand this behavior given he doesnt do anything like that to myself, my husband or my 16yr old. He also absolutely despises company over. Any help on this situation would be greatly appreciated. I really truly dont want to give up on Zeus :(