If he only hits (i am guessing you mean bangs his head?) against the pillow or mattress as he's going off to sleep, I don't really think you should worry.

But if he does (or starts to) it all night, at other times &/or against harder things then i'd get him to a dr or pedeatrician & ask about having a brain scan to check if there's something hurting him or something that's casueing him to do this. But if it's only going off to sleep & against pillow/mattress & can't see it as a real problem, maybe just a stage.