90 degrees

So, I have to take baby out in 90 degree weather today to go grocery shopping. I usually would have my mom watch him but she's on vacation. I plan on putting him in a tank top, cotton shorts, baseball cap and sandals. The inside of the store will be fine but when we come back out with all the groceries (I have to get a ton) the car will be hot inside. What can I do to prevent him from overheating?