Have your pediatrician check it at your next visit.
Often times, when a child hits a bone, it is similar to you hitting a young sapling with an axe, or carving one's initials into the bark of a tree, a scar forms at the injury site.
The bones are actively and quickly growing and the head in particular is not fused. My hunch is that your son's skull grew a protective boney callous over the injured bump location and it will take several months for the hard bump to be re-absorbed.
On a personal note, this happened to my daughter, around the 3rd grade, she was running up a flight of wooden stair, slipped and hit her chin bone. Her leg grew a very large boney bump. It was scary as the doctors thought it was a very serious bone cancer, as it had an active nuclear center, producing cells on it's own....but turned out to be her young bodies way of healing itself.