Talk to Pediatric Endocrinologist on Very Weak And Underweight

My son is 16 years he's 5'8 in height he's very weak and underweight his wheigh is just 50 kg He looks very skinny and weak we have visit multiple doctors they all said he's ok but his noticably weak we have also taken multiple tests they are normal (CBC,leaver function test lipid profile and thyroid h4 test all were normal but good cholesterol level was down) He is tired of following diet plans good diet but he's not gaining weight or strength