Chicken soup or chicken with rice soup or a very hearty vegetable and pasta or rice soup, nice and salty, goes down _really_ well later when people are sick of everything else but need "real" food and salt.

If it's going to be that hot, will he be carrying electrolyte capsules? That's pretty important.

For the days leading up to the ride- I'd just stick with normal meals- pasta, chicken or fish, rice. Less fiber the day before is sometimes advised so he will be less likely to have bathroom issues.