My Son Who Is 24 Always Feels Dizzy. He Gets

my son who is 24 always feels dizzy. He gets at times where he cant walk at all unless he is helped. he feels like he will fall always. If he gets real bad he looses his speech. He has gone to medical doctors and they say it is mental. He sees pyhciatrist and they have no idea. he has been on so many different medications and nothing helps Sometimes in bed his legs will keep shaking uncontrollably.. Please we need help