Glucotabs are a must with exercise... a very intensive sport can drop you quite quickly, and if you're concentrating and really going for it you can go quite low before you realise.
Banana's are good for me, sounds odd, but rice-crispie cakes too. Bit of chocolate, lots of dried fruit, and they tasted sooo much better.
The most important hting is to make sure that he enjoys whatever he has...
It's no good saying x is great, and y will work, because if he doesn't like it he just won't eat it.
Biscuits, granola bars, anything that's carby that he likes.