My son is 10 years old and only ways 40lbs

My son is 10 years old about 3 ft talland about 40lbs his older brother is about 4ft 5 and 75lbs.  The fact that he is short is not the issue as I am only 5'3"and my wife is only 5' so are kidsare not going to be giants.  My concern is his ability to gain weight, we have been to the doctors since he was very young being concerned about his weight, his last trip to the endrocronologist did not bring any new results other than his TSH level was almost to the 3.0.  My main question is at this age does testosterone play a big roll in growth and could this be his problem?  He also has ADHD and is on Concerta which would normally cause weight loss but he has not lost just never gained even before beingput on the medication and his need for it outweighs the risks.  Any insights on what is going on or what should be our next move.

[AnnieBrooke]

He is small for 10, if he is only three feet tall.  Let your doctor be your guide on that part of the question.  I don't think 40 lbs. is too light in weight for a kid who is only three feet tall.  My son is 42 inches tall and weighs 43 lbs., and he looks about right in terms of the weight to the height.