This is an extreme reaction to a canker sore. I've frequently had such sores on my tongue and do now that talking can be painful. So can eating. Is he eating alright. If he's eating then I suggest the not talking is not related to the sore. How long has this been going on?
I suggest that you back off and not pay any attention to his not talking. Tell him that you know he will talk when he's ready to do so and then don't mention it again. If he needs something and you don't know what it is calmly tell him he'll have to tell you what he wants because you don't understand. And then don't pay any more attention to him.
He's getting attention by not talking. If something is bothering him he will be more apt to tell you verbally what it is if he's not getting attention by not talking. If something is bothering him, getting attention is probably a part of his need related to what is bothering him.
Never make a threat that you don't plan to follow thru on. He will figure out that you won't do what you threaten and your threat will not only have any meaning to him but it's going to teach him to not trust you.
I suggest that reoccurring canker sores, which are a virus, are an indication that his immune system is weak. I suggest that you start him on probiotics. If he isn't already consistently taking a multivitamin that you also start those. If he's not getting 10 hours of good sleep/night then improve his sleep time.