My boyfriend has noticed he has a rash on top of legs and above his pubic area, what could it be?

He must of worn something he is allergic to. Or took something that he wasn't supposed to take. Or put some lotion or something. Or does he shave? Maybe that's it.

If it's just an allergic reaction, keeping it clean and dry and exposed to air and putting cortisone cream on it will help.
If it's spreading, it could be cellulitis and needs antibiotics. That's a staph or strep bacterial infection that enters a cut in the skin.
If he isn't sure he needs for it to be examined.