don't turn him! crack the back windows for some air flow (hot air will get sucked out... then, when the whole car cools, you can close your windows.
I would see if you could swing a remote starter... if your windows aren't tinted, get them tinted (check what the law says about how dark they can be first, though)
And then... my favorite summertime rearfacing solutions are as follows
1. Ice water - and I MEAN ice water... pack sippies (or bottles, but he's older, so I assume sippies) with LOADS of ice OR fill 1/2 full and put in the freezer till it's ice, and then add water when the time comes... pack in an insulated bag with an ice pack and make sure he has one at all times (when you are out, most fast food places/convenience stores will let you either for free or for very cheap fill your child's sippy/bottle with ice and water if you run out and it's a great excuse to buy yourself a nice big cold drink (cause then if you are able to fill it yourself they REALLY don't care)
2. Clothing - less is more... even once he's 2, let him be in just his diaper and a shirt or onesie... you can add shorts/bottoms when you get where you are going... heck, let him ride in just his diaper if he's cooler that way (but use plenty of sunblock)
3. WATER - I know I mentioned it before but sweating (and even redness to an extent) is not worrisome as long as he's drinking drinking drinking and it's WATER... if he won't drink plain water (and there are few kids at that age I've seen who won't, but they're there), mix in the TINIEST amount of whatever will get him to drink it to flavor it... Sweating is how the body cools itself and is a sign the body is hydrated... so it's not something to worry about if he's drinking plenty of water.
4. Hot air rises, cool air sinks... aim those vents straight up to the ceiling and blast them... leave them pointed up until winter.
We only live in virginia but the humidity here is special and our temps get that high once summer has really gotten started... my daughter was born in march, my younger son in July (and during an indian summer in PA at that), so I know how it is... with my son at the time our car had no a/c, so we just put all the windows down for air circulation and tried not to go anywhere we didn't have to.