I don't have a specific recipe for you..am searching for many healthy sorts myself to lose weight and get my fruits and veggies in. But I would suggest looking for some with pumpkin, zucchini, carrot, banana, etc in them. Maybe some boosted with fiber and you can use Splenda instead of sugar (we do) for baking. You absolutely can not tell the difference, I think. We do alot of splenda use..in coffee and teas, etc. I suffer from severe migraines and have been told by my Doc not to ever use "pink or blue" packet sweetener..and real sugar is just too many calories for me (trying to drop weight).
If you can't ensure hes eating well at school, but seems to not be suffering any "ill" effects (falling asleep in school, behavior or negative grades, etc)..I would pack him full of good stuff before he leaves and make sure he gets a good snack when he gets home.
Also make him part of the lunch making process. Get him involved..he may be more likely to eat it? Ask him what he has time for, but give him choices so the options are healthy. A string cheese or marbled cheese stick with some whole grain crackers and some fruit he will actually eat is quick and easy and fulfills the nutritional balancing a child should have to fuel them in the middle of the day. And again...send him out the door well fortified and do the same at home at the end of the day.
I do home childcare and when I still took school agers..boy oh boy., they were starving when they got home at 4pm. Most ate lunch at 11:30 am, with no snack after that. I started serving a "technical supper" as per USDA food program standards since they were asking for more than the fruit and crackers I generally offered . Add a yogurt or cheese sticks (real cheese)..and another fruit and it was a "supper".
Good luck!