I would give this one more "nice" shot...but make it "official"!
Go knock on their door, with a dog in arms, and pleasantly ask to speak to the child AND a parent together, if not both parents. Nicely (again!) mention the problem you've had, and that another hole has been dug. Since parents get defensive of their kids awfully fast, don't approach it like, "your rotten kid....", approach it as, "I know how kids are...." And then nicely (AGAIN!!!) offer that he can come over and visit the dogs at your house anytime he wants, per your convenience.
THEN if it's still a problem after all that....it would be time to get ugly!
