I am fairly certain my dad might have dementia, what should I do?

I'm fairly certain my 62-year-old dad has some form of dementia (he has many symptoms), but I don't know what to do about it. He's been getting more volatile in recent years, inappropriate and worrying. He takes the thought of mental decline as a personal insult, and he's extremely stubborn. I'm not sure there's any way to get him to bring it up with his doctor (I would just want him to get checked out and cared for). And I'm afraid to call his doctor, because I'm worried he'll be violent if someone raises a concern. He has a large gun collection, unfortunately. He's retired and I and my mother live with him. He's pretty unstable at this point. What should I do?