As a 21yr old Male who has had several girlfrienda😎 the best thing you can do is just tell him. If he is as great as you say he is then he is not going to care, and if it is a deal breaker then he's an asshole who missed a chance with a lovely girl. Honesty is the most important thing you can do when it come to these things, no matter how big of an Insecurity it may be.