W.C.
When he hits his head, stop him and say something like, I know that you are......., but it is better if you hit this big nerf ball (or something like that). Keep is real simple. And find something that is small enough for him to handle but makes a big enough wush for him to feel satisfied.
You could also get him a football helmet, but I think this is just a phase until he learns to use words.
Speaking of words, you could tell him to yell when he is upset rather than bump his head. It is noisier, but safer.
Good luck.