To do a gluten free diet, you will have to read labels and possibly make a lot of unprocessed food. You have to eliminate all wheat which will be clearly labeled, rye, oats, and barley which can be listed as malt or hidden in natural flavoring.
For breakfast, you could do eggs, grits, most sausage or bacon (read the label), some yogurts, milk, juice (but watch out for natural flavor), Fruity or Cocoa Pebbles cereal
For lunch, lunch meat and cheese, fruit, potato chips (check the label), Fritos, Cheetos, some fruit rollups or fruit snacks, Jello pudding, Tyson chicken legs
For dinner, meat (but if already seasoned or marinated, read the label), rice or potatoes, veggie
Also most ice cream, a lot of candy, popcorn, hot dogs, hamburgers
Those are all easy, regular foods. If you end up doing it long-term, you can make bread, cakes, cookies, waffles, or pretty much anything. A lot of the baked goods that you can buy are not very good, so I wouldn't start with those or you could scare him off the idea.
We saw a change in a few days for my daughter but I'm not sure it is always that quick. It sounds difficult but once you figure out what he can eat, it isn't that bad and if it works, you will not mind the extra trouble. You can read more about the diet at
www.glutenfreeforum.com.