Dont concentrate too much on it being your fault, unfortunatly it happened but now just concentrate on doing the things its going to take to make it better. Accidents happen and we are only human, we cant watch everything every second. I would say it really depends where on his tongue the cut is and if it did any nerv damage whether your going to be able to ride in a bit or not. But JD said, its really a question for your vet. I hope everything works out, keep us updated!