It does sound like a bruise ,is he knocking himself against something, any nighttime habit of rolling onto something hard. If not it seems to be that asking the Doctor what he thinks is the best way to go..Good luck
no hes fully white hes australian and no there like a browish yellow colour and its in the middle of his back and hes only started to get it a few months before his second birthday.
Are they blue/grey looking marks at the base of his spine , it could be a birth mark called Congenital dermal melanocitosis ' he may have had it since birth ,has he a darker skin.. American native Indians and Polynesians can have it ..