Yes, being out in the heat can make CFers ill. They sweat out so much salt and then their body is even more depleted then usual. We used to live in AZ and unless the boys were in the pool or going from the car to the house, we never went outside during the summer. Definitely get him some good electrolyte drinks and salty foods.
The Phoenix Children's Hospital CF center is very knowledgeble. I am assuming that that is where you are getting the sweat tests at. If not, that is where you should go. It is definitely time to do a full panel Ambry genetic test...to get to the bottom of things!!!
blessings,
missa