If you are going to have him outside make sure you provide lots of shade ( lots of plants) and lots of water. They can overheat. There is no "set" temperature. I always make sure they have plenty of foliage to cool down.
Yes. It depends on your chameleon what "too hot" is. If it's a veiled or a panther, I would say that 100 degrees is the maximum and I wouldn't want to leave him out for hours in that heat. For a Jackson's or a Fischer's 90 would be the maximum.