A little below average....

My boyfriend and I have been together for a few months over a year, and we've been having sex just about every time we see each other (Which can be difficult, because he's only eighteen and his parents can't know (I'm nineteen)). But as much as I hate to admit it, he has a smallish penis. It's not super small, but it's just below average. Sometimes I don't mind so much, like when I'm giving him a BJ... But when we are having sex, sometimes he struggles to keep it in, or it just doesn't feel like much to me.... He's the only guy I've ever had sex with. We both lost our virginity to each other, so we are learning along the way. We've tried lots of different positions, but it can still be difficult.... Any advise??