@lots33
Up until recently the best sex I had ever had was with a man with a small penis who had trouble getting and staying erect. He was, quite simply, the best at oral sex and other ways - the first person to give me multiple orgasms. He was a lovely man and a great lover. I broke up with him for reasons that were nothing to do with his penis.
My long term boyfriend before him had a big penis... but I had faked orgasms with him for most of our relationship. He wasn't interested in any foreplay beyond enough to get me wet enough to enter.
Another lover couldn't keep it up long enough to penetrate very long, because of watching too much porn, but we had an amazing sex life, he was brilliant with his hands!
Current boyfriend I haven't had sex with yet, but he's worried about the opposite problem - he's been too big for people before, so he is quite unconfident for that reason. I've done enough with him to be confident that even if worst case scenario we can't enjoy PIV sex, I'm going to be one happy lady, he's a very good and generous lover.
What I'm trying to say is that whilst penis size does matter, the men who have been wonderful lovers to me have been various sizes and included small penises and penises that don't work very well. What they all had in common was an awareness that female sexuality often wasn't all about penetration and they all cared about giving women pleasure. If you teach your son to be kind and considerate to his girlfriends (I'm talking in general) and, well everybody in life, and he translates that into the sack, then I'm genuinely certain that he won't have a problem being sexually compatible with somebody.
I think the bigger problem for coming off age teenagers is porn. It's not educational, it's created for male pleasure not women, and more extreme stuff is taken as normal. Keep your kids grounded and away from that stuff, as long as possible, and let them know it's nowhere near what they could and should be doing and expecting in real life.