Specifically the DD 214-4. That will tell you what his discharge is. He has a copy.
Reasons for receipt of anything other than a Honorable Discharge usually include some kind of Uniform Code of Military Justice (UCMJ) action that was unfavorable. This can range from adultry, to DUI, to theft, or any other violation.
I have had people try to tell me they received a Less Than Honorable for injury or disability. It seems to be the common excuse. However, I've never found anyone who could prove that (usually they just go away and I never see them again). If he did receive a disability rating that also will be on the 214.