My 8-week-old puppy fell on concrete. What are head injury signs?

The first question to answer is, if he lost his consciousness when he fell. Being 'knocked out' suggest brain trauma, concussion at least. If he was conscious at all times, then most likely the brain did not suffer any serious injury. Dogs have relatively small brains (compared to human), which are also well protected by the skull bones. Brain injuries are much less common in dogs than in human patients. So, my advice is : as long as Milo is looking normal, he is bright and alert, he is not vomiting, his pupils are of equal size, he is not in any obvious pain or discomfort you should monitor him at home. If he becomes lethargic, weak on legs, crying for no reason or vomiting you need to call your regular vets or your nearest emergency service asap.