Did he possible injure his neck with the concussion? I have an injury at C-6, C-7, T-1 and T-2 that cause my bowels to dysfunction. Sort of a TOS (Thoracic Outlet Syndrome) It is called paralytic ileum. My physical therapist can help me get things working by manually manipulating the ileo-cecal valve (externally). When it is acting up, I am like a moose with droppings rather than a single large bowel movement. It can be real annoying.

A chiropractor can sometimes help get these nerves working better. It depends on the skill of the chiro. No all chiros are created equal.

Maybe a physiatrist (Physical Medicine and Rehabilitation doctor) would be better to see. I would think they deal with this more often.

Either way, sounds like he needs some help.

My best to you.