Okay, well your girlfriend is right, he is just a kid, probably a smart alec but you aren't going to achieve anything by letting it get to you the way it is. Do you have anger management issues? If you do, you need to do something about that before you get much older because I can only warn you that it will get you into serious trouble and the last thing you want or need is a police record, right?
Why would you feel like a fool when you see him? You haven't done anything, he is the one who looks like a fool, not you. So now you being the older boy, you have to show him how you treat your girlfriend is the right way and not the way he did. He was probably trying to be a big man in front of his mates.
If you think you can speak to him maturely about it and not get physical and angry then try and speak to him about it. But please heed my warning about the Police record.
Keep posting and let me know what you decide to do.