I don't know about prescribing anything, but... difficult child is on a medication with an appetite-enhancing effect. Best things we've found are to actually PLAN for snacks - don't ever let the blood sugar get low, use a mix of protein, carbs and fat plus fiber at every snack. Keep snacks small, so that if he needs something every 2 hours, it's OK. Low blood sugar amplifies hunger really fast - for all of us.
And then... this is harder if he buys lunch at school, but... we found it helps to change how everybody in the family eats. We severely ration meat, and sweet, moderately ration complex carbs (grains, potatoes, etc.), and give free reign on fresh or cooked veggies. difficult child has gotten so he will eat a basic, balanced meal of moderate size, and then fill up on salad.