yeah, he probably likes her and is jealous 'cause he didn't have the guts to take her first, and that you got her instead. Either that, or for some reason he's got the notion you're bad news for her - and he's doing the "protective older brother" role some guys have.
It's much more likely to be the first option though.
-make sure she doesn't have any feelings for him, or it could turn into a problem later on. You don't want three people in the relationship man.