I've heard of people getting these sort of miles/kilometers out of rear tyres, I'm absolutely stunned at these figures.
The best I have ever got out of a rear tyre (Bridgestone) was about 5K Miles.
Welcome to the site from New Jersey, USA, Captain Roscoe.
I was the very first Rocket III rider to use the B'stone exedra max on the rear, however I've never gotten more than about 8500 miles outta' one. Your son must have doing a lot of riding on superslabs?