There is nothing magical about the term concussion or concussed. He has subjected his brain to injury. He has probably had many head impacts from sports and been able to continue to play. He may believe that not losing consciousness means he was not injured. This has reinforced his denial of being injured.

You could just ask him about his brain health. "Aren't you concerned about the long term effects of your head impacts ? Do you know what CTE (Chronic Traumatic Encephalopathy) does to the brain ? Do you know that all head impacts, even those that do not cause concussion symptoms, can combine to cause life long brain dysfunctions ?

Keep in mind that if he is a denier, you may be talking to a brick wall.