It gets pretty hot down here in south Alabama during the summer. I've found those cheap plastic (hard sideded) kiddie pools you can buy for about $15 at places like Wal-Mart to be extremely nice. The dogs love them. They can all easily get in and out as they please/feel the need to when they get hot. And if I feel they are too hot, I can easily tell them to get in and "chill" for a for minutes.
You could also try games with water hoses. Some dogs like to chase the water. I agree with those that said to replace the ball for training time. If you do play ball, I would hose him down before, during, and after the game of ball.
And of course, make sure he has a constant supply of fresh, clean water.
You mentioned you have an above ground soft sideded pool. We've got one left over from last year that I'm going to be using as a dog pool for my dogs this year. I plan to take the ladder that it came with and attach a ramp to it so the dogs can climb in and out. Then make a flat piece inbetween where the handle bars are. Not sure how I'll make the entrance to it so they can get up the ladder, but it's an idea.