Can a leopard change its spots?

During the four years that I have been with my fiancé he has cheated on me four times. I have discovered this by going into his business affairs.

The last time it was with a girl he had met on the Internet. When I confronted him he cried and promised he would not do it again. He now treats me a lot better and says he wants us to get married but I don't know if I can trust him again.

Do you believe it's possible for a guy to change his behaviour and stop cheating?