Talk to your DR. However just a piece of info I heard recently about boys/Men in General, Some are Growers and Some are showers. Its about 50/50
is his foreskin retractable yet?if not the penis can seem smaller than it is.if he is retractable then this could be buried penis or micropenis.
he is probably embarrased about this but if he has micropenis or buried penis a doctor could help him with these.
You measured his penis???
I find that odd, but my son is 7 and his isn't much smaller, so I would have to say no thats not normal, but I am not a perfessional.