Could it just be a development of puberty? My DH is constantly too hot so now we have a young baby and need a warm room he's actually sleeping in the spare room in just boxers with a fan on. I'd freeze like that! He's quite a big guy, hairy (might matter for insulation??) And generally just runs hotter. DSS is the same. Could he sleep under just a sheet rather than a duvet? Might be worth seeing the GP just in case.