First, he should definitely include the concussion on his history, especially since he was seen by a doctor and the injury was labeled as a concussion.
I am obviously not Mr. Mullen or affiliated with DODMERB, but based on what you've described, I do not believe 1 mild concussion from 2 years ago is an automatic DQ.
From the Medical Standards for Military Service, among disqualifying conditions, it lists:
History of mild brain injury if:
(1) The injury occurred within the past month;
(2) Neurological evaluation shows residual symptoms, dysfunction or activity limitations, or complications;
(3) Two episodes of mild brain injury occurred with or without loss of consciousness within the last 12 months; or
(4) Three or more episodes of mild brain injury
So it doesn't sound like that would be an automatic DQ, but it is up to DODMERB to decide. He should just give all the facts on his form related to the injury and his recovery. I would include what you stated about no residual issues.