2 yo hit head today. He acted & ate fine. This was 6 hrs ago. A little cranky. Now, hes hard to wake. He will but gets really mad & goes back to bed.

Best to evaluate. A 2 year old with an acute head injury likely needs prompt evaluation to determine if they need any further studies such as neuroimaging (mri or ct scan). It can be hard to assess the mental status of a two year old so many institutions use this age as a cut off i. Their criteria for neuroimaging.
Loss of conscious? Did he suffer loss of consciousness even for a few seconds then? If not, is this his bedtime? Did he have his regular nap earlier this afternoon? These are the questions to be screen out first.
Should get checked. Likely to be fine, but if you tell me you have a young child that is not acting like normal after a head injury, i can only tell you that you should take the child to the emergency room to be evaluated asap.
Call your doctor. Is this sleepiness his usual because it is normally his bedtime now? Or is this unusual? If unusual, call your doctor to discuss.