To me, this seems like an awful lot of damage for a dog who's not doing anything serious. 3 fractures? It's obvious that his body can't take the abuse, and he's still very much a puppy. It sounds to me like he's doing more than his body can handle, and has been for some time. He needs to grow up and get strong before any more damage is done. What's your hurry? He should have a lifetime to learn to do what you want.
He already has arthritis that will most likely cause him pain for the rest of his life. It won't go away. If you don't stop, the problems will only get worse. I'm surprised that you would even ask the question of whether or not to stop with his training. Isn't it obvious that he can't take it?
Fractures and arthritis are not normal for puppies, so something has to be causing these issues. Even if it's his personal physiology that is causing the damage, he can't continue doing the activities that aggravate it. At least give him time to grow up.