Under weight physique of 9 year old kid.

My nine years old son is under weight only 23 kg and extremely thin physique . He has tauncil in throat .All blood report are normal .No any negative pathology observation except slight deficiency of vitamin D. He eats and we provide them all nutrition in food but it seems that lever is not absorbing the nutrition and body weight is not improving.at the age around 18 months a doctor prescribed anty T,B dose for nine months . We gave it without break but later it became problematic for his lever , Later during various chekups there were no sign of TB but there are few lymphotic glands .The boy is quite energetic and has good mental health but bones are not developing neither body mass is increasing.He often complains about joint pain .