ADHD was a real condition that's had its definition idiotically widened and is now overdiagnosed because of a desire by pharma companies to sell pills. Then once they're diagnosed people tend to use it as an excuse to avoid doing anything productive even though in actual cases of mental illness/personality disorders usually the best treatment is to slowly desensitise yourself to it by repeat exposure. You can match up almost anyone to the vague list of symptoms so unless we're talking about something that's actually causing him issues in day to day life drop it.
Autism is a different matter but again it can be a tricky one as usually if not diagnosed young he'll have coping mechanisms and will have handled it. If you really think he's autistic see if you can find out about his childhood and if he showed the classic signs like hating being touched (or being touched by strangers, by sharp things) because that'll both confirm it for you and will provide you with actual evidence he cannot deny but may not have considered. A genuine autist will probably be convinced by that.
Describe the symptoms you are observing.