Thanks for writing to us.
The very fact that he is admitted in the A and E Room , is under observation , he has been diagnosed shows that your son is in safe hands.the possibilities for abdominal pain
A)They've kept him cconfined to bed obviously due to head injury
would have left him with Cushing's
stress /Curling's ulcers which occur due to corrosion on the posterior wall of stomach due to collection of Stomach Acid or Hydrochloric acid.
Neutralising the acid with Anaesthetic Antacid Gel would recduce the Pain and heal the Ulcers.Also H2 receptor Antagonists(like Ranitidine),Proton Pump
Inhibitors like Lansoprazole
will prevent Acid Secretion and thereby Ulcerations.
In addition you have not indicated whether they have done USG abdomen and Urinalysis to rule out hematuria.The abdominal scans and urinalysis would ascertain whether he would have sustained any soft tissue injury
or to any core abdominal organs.