Hi.
Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concerns..
As per your complain it seems that pain and swelling over the jaw after being hit is not abnormal as it occurs due to sudden blow to the to the jaw leading to trauma to the soft tissues leading to swelling and pain..
Secondly in case if he is unable to open the mouth wide and the mouth opening is reduced, then it has to be taken care of as it can be due to
injury to the jaw joint leading to displacement and dislocation..
You need to consult a Pedodontist and get him evaluated and a Panoromic
x ray has to be done along with manual palpation to he exact rule out the symptoms and can be treated accordingly..
You need not rush to go to emergency room immediately and can wait till morning and see a paediatric
dentist..
You can however give him
Ibuprofen to relieve pain and inflammation and do cool compresses over the swelling..
Hope this information helps..
Thanks and regards..
Dr.Honey Nandwani Arora..