My daughter has had similar issues. If there’s anxiety, he might need combination of anti depressants which can also help with appetite. We do the cyproheptadine and added a pedicure daily. That has really made a difference for her with height and weight gain.
Studies show that most kids jump back to their height curve after a year or so on stimulants. But adding the extra pedicure has really helped my daughter.
Keep trying meds and get a good psychiatrist with adhd experience and combination med therapy experience. You are not alone and doing the right thing for your child, keep exploring and this forum is great for extra questions.