He just stared and stared for a moment. He'd never thought. He didn't think! But of course, he could see it now, it was obvious, who this boy was.
"You're his son," he said in a stunned voice. That...that was wonderful. The Master had a son. Amazing. He shook his head to clear it. "What do you know about alternate universes?"
An abrupt change in topic, but best to clear up the confusion, so they could talk sensibly.