My 6 year old has only gained 4 lbs in 2 years? I need advice please.?

I would take him to another doctor. At 34 pounds he is under the 5th percentile for his age. He is right at the 5th percentile for height though, so it may not be anything to worry about. Especially if he has always been at the 5th percentile or lower. He may just be small for his age. It seems as if he is eating ok. Does he have lots of diarrhea? That can be a sign that he isn't absorbing the fats that he is eating. Especially if the diarrhea seems to be oily. Not absorbing fats and being thirsty a lot are both symptoms that need to be looked at.