My grandson fell backwards and hit the lower back part of his head and it swelled into bump right away, should we take him for an x-ray ?

Need more info...: If your grandson fell and got a swollen area but cried immediately, acted normal, isn't vomiting, and is acting fine now then there isn't likely a need for further evaluation. IF there is any other issue, concern, behavior change, or worsening of the swelling have him seen. If you are concerned - an evaluation can also be very helpful to allowing you to feel better yourself.

Closed Head Injuries: Closed head injuries in children are relatively common due to their activity level and altered center of gravity. Most serious head injuries are associated with loss of consciousness, seizure and immediate vomiting. Serious injuries rarely become apparent within 24-48 hours after the injury and are associated with multiple episodes of vomiting, altered mental status, seizures, etc,

Head: X-ray not necessarily, being evaluated for it yes. Common area to swell up and more than likely should be ok, but let a medical professional evaluate it and determine next step. Likely observation will be the right approach unless develops any vomiting, major change in behavior, etc.