I don't know. It's hard to tell from what you describe and without seeing a picture or something. The head can have all sorts of bumps and shapes and be perfectly normal. If you think that there is some significant change in the last days where he might have had an injury resulting in swelling or the area is discolored, then take him to the doctor.

If he seems to be happy, eating, growing, and gaining weight, then there is probably no cause for concern.