I’ve got a skinny 12 year old.
Before you add more ingredients to the mix, try adjusting your eating and meds schedule.
Eat before taking meds. Lunch may be a complete loss. (it’s ok).
Expect to eat/snack at odds with others and late at night (it’s ok).
For what it’s worth, we felt the loss of confidence from being small and weak, was a greater risk than undesirable behavior.
All the best,