I've had a mole removed when I was around 22 or so. It was on my neck and it was raised so I kept accidentally scratching it, rubbing it, it was often exposed to the sun etc. and it started to modify, it had more than one colour, it had a mildly irregular border etc. It was small, like around 5mm, but one day my family doctor noticed it and told me it looks a bit odd and I should remove it, which is what I did. Although it had changed, the biopsy revealed it was very mildly dysplasic (don't remember the exact term), basically it was nothing. I could have kept it for another 20 years she told me. But it was better that I took it out.
I'm sure your son's biopsy will show a similar result, don't worry.