My son (I'm his father) is 15 yrs old his penis girth is a little under 4.7" around is he small average or big for his age?

Apparently average. According to Wikipedia, the average penile girth when erect is 4.8 inches. However, that varies widely depending on where it is measured, how much erection, and who is doing the measuring! The best thing you can do for your son is to remind him that he is NORMAL, just the way he is. Being happy and healthy is more important than size of penis, height, color of eyes or any other feature.