After my concussion, my first symptom (and the most severe for the first several weeks) was nausea. Is he nauseated or having stomach pain?
Also, if he was taking a lot of NSAIDs for headaches, that could affect the stomach.
There is also a TBI/PCS board on these forums that is VERY helpful. Come on over and join us!
http://neurotalk.psychcentral.com/forum92.html