K.H.
Are you concerned about the not holding his head up yet , or that he put his head back down? Either way I think it is fine , you have to allow for the fact that he was early , my 3rd child was 6 wks early and she was around 8 wks behind on developmental milestones. If you really are worried then there is no harm in checking with your doctor , but if he is happy in himself and doesn't seem in any obvious pain or discomfort then all should be fine.