just ask him about it, in a friendly way, when you are together, but don't make it obvious you're jealous. be the bigger person and remain calm and it'll be better because then you can ask his reasoning. that way if you are prepared to forgive him, you won't be too worked up and won't say something stupid. if you choose to dump him because he's a douche, then you're calm and he can't accuse you of 'not thinking straight'. you seem the better guy and can kick him to the curb.