My Fiances cat is attacking my 18 year old son!

My fiance just moved his 5 year old, neutered cat into my house, I also have a 5 year old, neutered cat...We have kept them in seperate rooms that have a glass door between them so that they can see each other, and also switch rooms so they get used to each others smells...That part is going fine....but.....my fiances cat attacks my 18 year old son everytime he sees him, even if he just walks through the room. (no, he has never been mean to him. he tried to pet him and make friends with him but he attacked him everytime so he's sort of given up) Also, even when my fiances cat (Midnight) was in his own home, he attacked the real estate people that come to show the house and they told my fiance he had to get him out of the house or they would not show it anymore, so this is not just a new enviroment....HELP!! I'm worried someone is going to get hurt...

I don't have any advice from personal experience, but I know others on this site do/will. In the meantime, here is an article on Cat Aggression. Part II talks about some of the causes, and Part III talks about what you can do. Hope this is helpful.

Cat Aggression Toward People