Thank you for your query.
I appreciate your concern. It is very difficult to give an opinion without having a look at the spots, however, I shall try to answer your queries. As you mention that you are light brown and his father light coloured the spots may be the changing colour of the skin which does happen. It cannot be vitiligo
as vitiligo is depigmentation
of the skin.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.