He has had over 10girlfriends!?

well he choose you! He didn't have with them but with you. He is 21, maybe he just wanted to fool around a bit and now he is ready to settle down in a real relationship with you. If you think he is sincere in his interest in you then let it go. It is normal to be jealous and in insecure a bit but remember you have him now. Don't let it ruin what you have, if you cant get over it , try gently talking to him.