I suspect that applying ice to the head is dangerous because it might send the delicate brain cells into shock... ice slows down circulation... which is safe enough for other parts of the body but reduced circulation to the brain is not good.

Bumps to the head always look worse than they technically are. Just observe his responses and state of consciousness (for the rest of the day) and take him to hospital if there is anything out of the ordinary like complaining of a headache or drowsiness when he shouldn't really be tired.