Feeding a kid a decent diet can be tough. You can serve it or pack it, but will they eat it? (I lived through high school on ice cream sandwiches. My real lunch got tossed. I know better now.)
One way to make it more likely that your child will eat healthy food is to only stock the good stuff at your house. If chips and cookies are available, they will be eaten first.
Also pay attention to healthy foods that your child really likes and be sure to always have them on hand.
Kids with ADHD (in my opinion) benefit from lean protein at every meal. That’s easier to do than you might think. WebMD has a nice little list to get you started.
Another thing you might consider is supplementing their diet (especially if they eat ice cream sandwiches for lunch ). Again, WebMD has some good ideas.
The one thing I would add is Vitamin D. Most people in the US are deficient.
At the very least, consider a good multivitamin for your ADD Student.
Remember, a healthy brain is a better functioning brain.