We tried a latex mattress topper and it helped quite a bit with the heat buildup, and a friend of mine swapped to a tencel-covered cooling topper which she said made a noticeable difference for her daughter. That said, since he's sweating a lot even with aircon on, it might be worth mentioning it to your PD at the next visit just to rule out anything, because sometimes excessive sweating in young kids can be worth a quick check.