My toddler is getting a terrible heat rash from his graco car seat. Any solutions for helping stop this?

Lay a towel or soft blanket over the seat before you your little on in the seat. It may get wet or slopped on but hey its removable. Just adjust it so it is covering the spots that are getting hot. You can also keep it covered while the child isn't in the seat so it doesn't get hot in the sun and heat while they aren't in it.

When you get out of the car, drape a large towel over the seat so the sun won't hit it directly. Then run the A/C for just a minute before you put him in the car. Seat covers are great, and I recommend them - but the towel will keep the metal buckles from getting too hot, as well.

also try baby powder on his back to absorb the sweat and sooth his skin

If it were me, I would call Graco, especially if the seat is less than a year old. They very likely will have a solution for you. Is he still rear facing? At one year old, he should be. Children should stay rear facing AS LONG AS POSSIBLE! Not just to 20lbs/1year but to the height and weight limits of their seat. They are safest rear facing. That said, if he is rear facing, go to the trouble to start the car and run the air for a few minutes before you get in, so he's not sitting back there sweltering while the car cools down. If the car does not have airvents back where he is sitting, aim the front vents right up at the ceiling the air will bounce off and get to him faster. They even make car seat coolers, do a search on eBay. Or, fill 2 water bottles with water, freeze 'em and put them in his seat wrapped in a towel (to soak up the condensation so his seat isn't wet) when you go in somewhere. It will keep his seat cool and ready for him. Get some tinted window clings (check Target, $2) you can stick to the rear windows so the sun isn't baring down on him and his seat, just make sure not to get roller shades or anything with suction cups. They become flying objects in a crash and could really hurt the child. Get the static clings. First Years makes some. Cover his seat when parked in sunlight. Pat him down with powder, though make sure to use pure cornstarch, no talc, before putting on his clothes, that will help big time, and make him much more comfortable in general during hot weather. If not of that works, he may be having reaction to the material in that specific seat, and you may have to get another one.