You might want to search the net for "delayed puberty". At 15, he's on the edge of "normal" for beginning puberty but not outside the statistical range.
One cause of delayed puberty is genetics - he's just a late bloomer genetically. Your mention of another 14 year old son who is just now beginning puberty would bear that out - that's a little late too.
In general, is your son healthy? Does he eat enough? Is he free from chronic diseases like asthma, cystic fibrosis or diabetes?
Well as a young girl I blossomed late as well so it all depends on the person's body