Most people can stop technique when uke knows what nage is going to do.
Or alter their attack so that nage can't do the specified technique, or get their attack to "succeed" by putting themselves in harm's way and counting on nage's restraint (how many techniques have you seen "defeated" by an uke who was oblivious to the fact that nage is trying hard not to hurt him/her?), or otherwise interfere with nage's practice. In all likelihood, big fella is doing one or more of these things. It would be a kindness to him to point it out; otherwise, he's just not going to get any benefit out of his training (apart perhaps from a little shallow and short-term ego gratification).