Not on the list, but I say Wrestling.
I love BJJ and train almost daily, however, as a grappling base, I think folkstyle wrestling may be better, especially for kids.
First and foremost, when you’re young, you can recover quickly from the intensity of a tough wrestling practice. The cardio and strength you get from wrestling is superior to most BJJ. Anyone who grapples at a high level knows that a strong high school wrestler (or college) can bring a strength and intensity that is hard to replicate in BJJ. It’s a nightmare to deal with.
Also, BJJ’s greatest seeming weakness is in the realm of bringing the fight to the ground. If you start with that skill set in wrestling and add the body awareness that comes with it, transferring those skills to BJJ is easy. A solid wrestler with a year of BJJ feels like a purple belt.
Lastly, he’ll have the ability to participate in school sports. It may not be important now, but in HS being part of the wrestling team and traveling to tournaments and learning to perform under pressure will build character.