Trying to make me Jealous?

Not sure of your ages but it sounds like he's too young to be so calculating that he'd intentionally set out to make you jealous. You say he "admitted to liking" you, which sounds like he's not too secure in this relationship and maybe you're not visibly reciprocating his interest. Flirting with someone else is a way to keep some emotional distance, keeping it light so he won't get hurt too much if things don't work out. Very likely he's not aware of why he's doing this or the effect it might have. If you can look at it as an innocent defense mechanism instead of an evil plot, maybe it won't bother you so much.