Hey,
I'm sorry to take so long to answer you questions but I haven't been on the computer for a while. My son was only two years old so there is a big age difference and where I could fall back on baby foods and etc. you probably won't be able to do that with your son. Pediasure mixed with baby cereal and banana's warmed is all I could get my son to eat. Banana's are really high in sugars and protein and the cereal just adds some vitiamins and grains. Ensure could be used in place of Pediasure and it taste a lot better if you make a shake with it maybe with some icecream. Custards and puddings might taste good to him and go down easily.
I hope I helped some!
Hang in there!
Brenda E.