I love my boyfriend's stomach but?

He seems to be insecure about it. When he changes in front of me he tries to avoid showing it. When we have relations he doesn't take his shirt off and I sometimes want to take it off but I'm afraid he might stop me or I don't want to make him uncomfortable. He is not fat nor lanky he's perfect! How can I make him realize that? Because that is what lured me to him in the 1st place, his confidence! Should I just learn to deal with it because everyone has insecurities or is there a way I can make him comfortable with himself? Thank you in advance (: