Overall fit on the bike, and core strength are the two things that (usually) will help regarding numbness of the hands, forearms, and pain in the shoulders and neck. Commonly, it's because a rider is putting too much weight on their hands, and not enough on their backside and feet.
Raising or getting a shorter stem will likely help to alleviate the problem.
(Intentionally vague, because it's not possible to really solve someone's fit problems w/o seeing them ride.)