I live in a place that gets hot, humid summers (much less hot than where you're going though) which I really can't deal with, so here's a few things I do to try to handle it, maybe you can try some. Some are super obvious but I'll post anyway:
I soak my Tshirt and wear it wet, it drys pretty quickly but it really helps keep me cooler. I do the same with my hair, you could get a wet cloth or hat for his head if he doesn't have long hair.
I spend as much time in the shade, of course, and I do things like partially filling the bath with cold water to cool the flat. The bathroom is lovely when I do that! So if do this in the hotel or whatever you are staying in then at least you will have a cool place for at the least night time so he is able to sleep a bit better. If I get a good nights sleep I cope better with the heat in the day.
I never open my curtains in full summer, I know it's habit to open them but it's important to keep the indoors as cool as possible.
Drink at least four litres of water a day. I use glass bottles and keep the three on not using in the fridge well in advance so they are cool.
Don't eat bit heavy meals as I can't digest them in the heat haha it makes me feel sick. Watermelon and salads are the way to go, and lots of small meals, never a big one.
I can't as I'm allergic to perfume, but you can get suncream that's like a mousse which would mean none of that constantly moist feel that most suncream seems to give (or is that just me?).
Wear white clothes that are baggy and touch as little skin as possible. I wear dresses all summer usually as even the waistband on shorts is horrible and ends up all sweaty and making me feel rubbish.
If you can, take him out for a swim in the mornings and evenings before it gets really sunny. Playing in the water or even paddling guided by you will get rid of a lot of energy. I go for morning jogs with my dog to tire us both out (wish I could go swimming somewhere) as otherwise I have this weird mix of energy but too hot to do anything as it warms up during the day which makes me really grumpy haha!
Anyway there you go, some of these will be no help at all, some might be useful or might make you think of something else. Hope it helps and I will be eagerly awaiting other posts so I can get more tips for myself! Where I live it gets up to 35 (sometimes more) in summer and I'm already finding 25+ unbearable haha. I'm a snow/rain lover! 
Have a great holiday if you decide to go!