Did the man try to convey his own message using the new-found communication mechanism?
If I understand correctly, he was given the means to convey "yes" by imagining himself playing tennis. The concept of "no" was encoded by the absence of "yes".
If he had wanted to send some message, like "tell my children I love them", or "I feel pain in my abdomen", how would he encode it?
He certainly can't send messages as complicated as that, but if I only had one bit to flip, I would be flipping it as much as possible to get someone's attention.
That said, he probably did the correct thing by simply answering their questions. That shows that "someone is home" better than flipping his bit randomly.
If I understand correctly, he was given the means to convey "yes" by imagining himself playing tennis. The concept of "no" was encoded by the absence of "yes".
If he had wanted to send some message, like "tell my children I love them", or "I feel pain in my abdomen", how would he encode it?