He's young, it's not too late to teach him NOT to do that. Much simpler than it might seem. He runs the fence, tell him NO and if he doesn't stop, he has to go in the house for time out. Repeat often until he knows what 'knock it off or you'll go in the house' means.
All my dogs in the spring would start fence running because woohoo there's more people and dogs out and it's just so much fun. It's like they forget over the winter. Even my last female, at 10, she still had to be reminded, run at the fence, it's game over you go in the house.
If your dog's fence running with another dog, that's a whole other PITA...
Venus of Crooked Creek 1/26/13
Otto von Hena-C 05/23/08
Morgan Donnermond 08/04/01 to 9/28/12 Long Live the Queen!
I have duct tape and I am not afraid to use it.