Often times when people see a spur is missing, its actually not. A few bad sheds could make a spur look non existant. make sure your snakes shedding process is going good. Hopefully the spur is actually still there.

Then again, it could have been caused from some sort of Trauma, I wouldnt be extremely worried about it, and I am sure he could still breed just fine. If it worries you bad enough, you could take him to a Vet and clear your worries up there. Good Luck to You.