Diagnosis for my brother

[Muguyaro]

For as long as I can remember my brother has always had an overactive imagination. My parents believed that this was due to him being a child at the time and that he would grow out of it. He is now 22 years old and things have gotten worse. His behavior now includes repetitive pacing, talking to himself, zoning out into his own world and self isolation from social settings. I have approached my parents about this behavior multiple times but they refuse to believe it is a mental condition. When I talk to my brother about he either brushes it off or refuses to speak about it. My parents always tell him to snap out of it and get himself together. This usually results in yelling and yields no results. It would be appreciated if anyone could please help me convince my parents and my brother that he should seek professional help. If possible giving a diagnosis would also be appreciated. I have done my own research but I'm not a medical professional. I just want to help my brother out.