I have a 2 1/2 year old right now too. She has hit her head enough times for me to consider a helmet (just joking... sort of) a few of them had me worried enough to call the nurses hotline and get talked through all the signs/symptoms to watch for after a head injury.
One time she hit her head on concrete and threw up immediately after (I guess it was from crying, she was fine, but still scary!)
Another time she got one of those gigantic purple goose eggs on the back of her head (which are supposedly harmless but still scary)
And once she fell down the stairs and immediately seemed to need a nap afterward (just happened to be naptime, but still scary)
... and those are just the ones I called the nurses about. Anyway, here's what I've learned: when I am worried about a head injury I don't let my child fall asleep right away because then you can't monitor symptons (until bedtime, when I know I haven't seen any symptoms by then). Watch for bruising under the eyes, behind the ears. A bump is fine, a dent is NOT fine. pupils different sizes is not good. listelessless, not self, can't stay awake is not good. Throwing up more than once is not good. Incoherent speech is not good.
I'd say if your son is back to his normal self he is probably fine.