Sounds like a guy that can't get a hard on, and then takes it out on the females.
Key to a strong relationship is communication so with that being said; ASK HIM.
There's a million reasons why guys are unhappy.
He could have any one or more of them as reasons why he mistreats his girl friends.
Being a closet homosexual is only one of them and not the most likely at that.
Since you're his friend, why not ask him.
Maybe he really wants YOU!
Well if you think he's gay why not support him as his friend and help get him started in a coming out support group. Maybe you know a friend who knows a friend that is gay and set him up on a date. Make it in a place out of town so he feels more comfortable about the whole idea like a gay bar. You will make him so happy if you are right.