id be a little concerned too,but i dont think the dentist will be, theres not much you can do apart from requesting an xray just to check to make sure there are indeed there!! id say there will be he's jsut a very late teether! willget his adult teeth through very late probably as well! a friend of mine her daughter only gor her adult incisors (front teeth) at around 8 yrs! she was also very slow.
its usually genetic, aske your mum or mil if you or DP were last teethers, you may get you answer from them!!