Hello Good Doctor, My Problem Is On Growth. My Younger

Hello Good Doctor, my problem is on growth.
My younger brother happen not to be ageing, we are confused what might be his problem, I know you're confused too but the problem with him is that he has no facial and body hairs, his private part(penies) is very small. We've done series of test but no good result.
Pls any answer to help will be highly appreciated.

Thanks

Benedict