R.J.
This happens all the time. TMI to follow:
My husband has about 2 inches of puckered scar line that runs from the base of his head down his shaft. Many, many men have similar (ranging from 1/2 inch to several inches). For some men i've known, it creates "bends" in their penises... where the scar didn't stretch evening.
Scar softening cream, applied daily until the end of puberty. Yup, for the next 20 years or so. Make sure that it is applied at night as well. All boys from infancy on get erections at night during certain phases of their sleep AND when then need to pee. So night time is the absolute best time for it to be slathered on good and thick.
In extreme cases, surgery is warranted... but usually scar softening cream & lotion.
It's not the end of the world. AND The upside for your son, is that he will actually have a medical reason to masturbate as he gets older (because obviously the scar has to stretch more when he's hard).
ALSO you're lucky. Many boys don't find out until they HIT puberty, which is painful. Being able to start so young, the scar will be nice and soft by the time he starts to gain some serious length & girth.