"My seven-year-old hits other kids when he's frustrated."
Michele Novotni, Ph.D.
Say something like, "I'm terribly sorry that my son hit yours. He's a great kid, but he doesn't deal well with frustration. He's working on self-control." Ask your son to apologize to the other child, and follow up right away with a consequence—such as a time-out—so that the parent will see that you mean business.
By the way, you might want to avoid the activities or locations that have proven especially difficult for your son. It may seem obvious, but it's also important to supervise him closely, so you can intervene before there's trouble.