Help!

My bf is insecure about his dick. Which doesn't make sense to me because he makes me cum whenever we have sex. I've told him plenty of times that size doesn't really matter and it's about the connection/love/bond a couple has when it comes to bed. 
I mean it should only matter if it was a hookup? But WE'RE DATING. How do I advice him that he has nothing to be insecure about.