Feed him lots of fruits and veggies - just don't feed him anything with Sugar added. A natural occurring sugar, such fruit sugars won't be so bad.
Is it possible to make some muffins or cookies using whole wheat or spelt flour? Use molasses or honey(unpasturized) to sweeten. Use your own discretion. Only you know what is available to you, I just suggest strongly that you read all the labels and be aware of what your son is eating.
You have a good handle on this and access to a lot of great information...and his strong appetite is a good sign!