My son has a bump on his head that's been there since he was about 6 or 7 weeks old (he's now 5 months). He didn't bump his head to my knowledge, but the doctor thought it could have been from trauma and said that sometimes blood can collect and calcify after a while and make a hard bump but it should eventually go away. Well, my son's bump turned out to be a cyst, but I think what your son has sounds like calcified blood from the bruising. If you are very concerned I would take him to the dr to have it looked at again, it could be possible that the bump to his head altered the way his bones are developing or that a tiny fracture may have occurred? Anyway, they can do an xray for your piece of mind. Be well!