I'm no expert, being child-free myself, but I've found when dealing with children in my care (the nieces), that asking them what they fancy to eat, then buying the ingredients to make it, rather than buying a packet version works well, as does deciding for them and buying it before they arrive. So, if they ask for lasagne, I buy mince, tomatoes etc and we make it together. If they want pizza, ditto. You have the control then over the ingredients. Other than that, don't go to the supermarket where they have tall freezer cabinets - take him to the smaller shops, or the market - or just don't go down that aisle!