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A dietician can work with you to create some individualized high calorie smoothies or baked goods he can eat (like muffins with protein and veggies in them).
Remember, it won't be the amount he eats but what's in what he eats. He may not be hungry all the time - but he may have a smoothie or two in the day plus a muffin for breakfast and casserole for dinner, but if they all have protein and starch, he may increase or at least maintain weight.
Your local yellow pages will have a listing for Dieticians - call one that either works with your insurance or get a referral through your pediatrician.