A few thoughts:
*Check out the Livestrong’s MyPlate website. Even if he doesn’t have the internet access or the commitment to keep a log, but it might be a good one-time activity to sit down and have a conversation about how calories add up, how exercise takes them down, etc. since they have a lot of good visuals. And if you’re feeling bold, there’s probably a math lesson in there somewhere with all the graphs.
*As a simple book he can carry around, maybe a day-by-day calendar and some stickers to put on each page? For example, he can put a green sticker each day for every time he eats a serving of vegetables.
*There are a lot of after-school programs that address healthy eating/fitness. If there’s one in your neighborhood or at his school, either encourage his mother to enroll him or see if the instructors will share some of their worksheets/activities with you.