Hi Liz,
just remember that all young boy have insecurities at that time, no matter what size they are. Don't worry about it too much, and if you let him know that everyone else in his class is probably worried about puberty and things aswell, having a bigger penis (or a smaller one, or a regular one) isn't going to change that.
But i think you're doing the right thing. If I'd kids, and i found out that there was something in their life that may make things a bit difficult, or even just different that i'd gone through, i'd want to find out more, so i could give advice aswell.
Well done and good luck
Gord