Go and see a chiro, preferably a paediatric chiro if there is one nearby. He may have a stiff neck which is why he keeps his head in one position.

My DS had quite a severe flat spot (basically the whole back of his head was flat). We saw a chiro for a while but at 7 months he wasn't really improving so we saw a paed who referred us to an orthotics clinic where we got him a helmet. He's had it for 4 weeks now and the difference is amazing, his skull bones are popping out and his head is getting rounder. We were told that if you don't get it sorted by 12 months then that is basically the head shape they will have. Don't want to scare you but if you have concerns then its best to get another opinion. Not sure where you are but at the RCH there is a long waiting list for helmets (and I suspect it will be similar for other public hospitals) so you want to get onto it now!

Hope that helps. Let me know if you have any other questions and good luck!