Okay, so I've been dating this boy for five months.

I'm quite fond of him (five months is a long time for me to be in a relationship looking at my past relationships lol)

The problems..
-He can't get it up, ever. He hasn't been able to stay hard and it sucks lol what can I do to fix that? He smokes a lot of weed... would that affect it? Oh, and he used to fuck his ex like.. ALL the time (I know this because before we were dating his best friend told me)

-He's shown up to work with hickies on his neck twice (one time he got "attacked" [but there were TWO on his neck] and the other was just a "normal bruise" from fighting with his friend)

Yeah....
so
yeah... so my theory is that he is cheating on me..... but would that make him not be attracted to me? I don't think I'm THAT bad looking...

But yeah... is there anything you would recommend doing to help him?

I don't understand it.. if he is cheating on me, why not just break up with me? Like, why stay with me.. especially if he's not even sleeping with me