Eldest Son had (has) a lot of food issues stemming from his Asperger's Syndrome. I always had to be prepared for him to be unable to eat his dinner and need to make up the difference with granola bars and crackers. It made for heavier food weights, and sometimes over-stuffed parents (as we ate his unfinished meals).
Whenever I do backpacking with my son, I usually explain him first about the duration of days we're gonna stay on the area and then talk about the needs(which he includes important) that he might think is useful for him during the bp. Kids are really picky and that's why I would let him choose the food he wants to bring. Let him test his sleeping bag if he's comfortable with it. Water is a must so bring a separate bag for water containers filled with water. hopefully, this helps.