Mine overheats as well.
Drinking a lot with a tsp of salt added to every 24 ounces helps a lot for him!
I have talked a lot about Nathan running unexplained fevers when he was younger, to the point that we almost went to the ER a couple of times, and found out that the issue was the salt intake. I would advise that if you are having issues with your little ones BEFORE any real heat has even happened, you should talk to your CF Team and get a dosage of salt that you should be added to drinks or food now. Mine perfers it in gatoraide because he can't tell or taste it in the gatoraide. Also, I would plan on the need increasing as the weather gets warmer.
I can't imagine telling Nathan he can't play outside anymore! Though, if there are a lot of fires and the weather is unusually hot, he does stay in more but otherwise, he goes to the pool, amusement parks, baseball games, park, camping, and whereever esle his friends are that he wants to go. We just make sure that we have lots of gatoraide from Sams Club and water with kool-aide packets and salt packets and away he goes.
This is one area that you just can't overprotect and stop them from being kids, you just need to learn how to adjust appropriately for them with liquids and salt.
Another thing that we have done when we know that we are going to be out where it is hot for a while is a wet banddanna that we keep two in the cooler to rotate and one on him which helps as well. They even have these really cool ones that all you have to do is wet them down and they get really cold and you wrap it around their necks to keep them cool and they work! You can usually find them at any sporting good stores, probably online too if I could remember what they call them officially. That with the liquids and salt are perfect for all CFers that have overheating problems, issues.
As anal and overprotecting as I am, I know that there is no way that I could expect mine to stay in all summer or whenever I thought it was too hot or too cold for that matter. Just have to adjust and do the best we can to help them regulate body temp.