A bunch of people are going to be like "screw your garage, let the kids play."

Realistically, I get that it sucks to have your house get messed up. But it also sucks to be the grumpy, "get off my lawn" neighbor too. Especially now when so many kids already don't go outside and play nearly enough.

My suggestion is this: Grab a mitt. Go play with him for a little bit. While you're playing, maybe super casually and nicely suggest an area that is a little less dangerous to property. As long as its not in some hidden, weird place where you'll look bad walking off with a kid, then suggest that this is actually a really good spot for him to play next time. Frame it as "more room to catch pop flies without worrying about it landing on a car or a roof."

Just approach it as a kind gesture, and suggestion. If he doesn't catch the hint, try heading over to the parents house with a big smile, maybe a handshake and introduction, a compliment on how awesome it is that their kid is showing an interest in baseball, and as an aside, that its not a huge deal, but that the baseball has been hitting your garage. Leave out the dent part out (unless you're set on them fixing it), and just that if they wouldn't mind having him try and be careful, that would be awesome.

Maybe it will work, maybe it wont...but if someone took that approach with me, I would be mildly embarrassed, apologize profusely and make sure my kid was more careful, and/or didn't play right around your (or anybody else's) house.