I sympathize with you trying to do what you know in your heart is the right thing, when there is so little or no pediatric medical advice to support you - and so much uninformed contrary advice from family, friends, teachers and anyone who comes in contact with your child.

My understanding, but do your own research, is that it takes very little of a substance for the digestive enzymes to form. Sweet potatoes or yams (they are not technically the same thing) are starchy yet far less harmful than white potatoes. Possibly a little of that once in a while would solve the enzyme worry? Oats and rice, the jury is still out even for adults. Some do okay others (like me) find them harmful.

No matter what you do, your son is going to be exposed to all manner of sugary, carby food that is going to taste good. Hopefully you will be one of those amazing parents who found a way to give him a firm basis of knowledge so he'll refuse more of it than he'll accept. It can be done. I know a stunningly beautiful, 19 year old girl (daughter of a model and a dentist) who never had anything sweeter growing up than an apple. She's slim, but very fit and healthy. By the time she could make her own decisions she preferred to eat the way her parents raised her.