This patient support community is for discussions relating to relationships, abstinence, arousal problems, birth control, cohabitation, commitment, communication, couples counseling, desire, sexual technique, and sexually transmitted diseases (STDs).
He's in his 40's so no, without a big committment on his part to change it won't get better.
My advice is to go look for an adult. Like jo said You sound very responsible and mature and I don't think you would be happy if his attitude never changed.
My friend was married to a man who cheated so she knows what it feels like. She would never do that to someone she was committed to; however, you cannot get that through her current husbands head.
Nor will you ever be able to "convince" your boyfriend that you won't do that. He will either come to accept it over time or he won't. But either way there is nothing you can do to convince him. Think long and hard about whether you want to stay in a relationship that has such serious trust issues. It's a very difficult road.
Good luck.