We still struggle with this for DS9. He's a skinny kid by nature and doesn't have muscle or fat, runs between 10th and 25th percentile weight for height. Every fall, he stops eating. The lunch room is crazy (>100 kids) and there are constant announcements over the microphone, not to mention trying to balance on that little circle of a seat with your feet dangling.
Every fall, I need to take a few inches off the waist of his pants, and every summer, I let them out. At 8, we finally felt comfortable letting him exercise (swimming), hoping the increased hunger would help. Before that, we were too much on the margin to risk it. He is finally getting healthier and not so easy to get sick now that he's eating better too.
Give him FAST food that doesn't take a lot of effort to eat. We do raisins in a box rather than grapes because they stay put better. Apple slices in a plastic bag, with a tub of peanut butter were very successful for a while. We also do water in a bottle that I have pre-loosened because the straws can take a lot of focus to get into those pouches or boxes and waste a good 5 minutes. We've done sandwiches mostly, but also granola bars half opened in a plastic bag. If it takes too long to open, it doesn't get done. Those are things he's told me over the years.