Welcome to HCM
We understand your concerns
Here, every parents are concerned about their children's health. Your son is 11 years. Child developmental schedule points that ages from 8 to 13 are most critical in their intellectual development and children uses brain as well as every body resources to gain intellect. Therefore during this age, weight loss
is normal across races. Don't panic. As you said he is frail, just check with a pediatrician
for anemic or malnutrition
Otherwise he should gain weight within another two years. good luck.