Haha! Don't worry at all. One of my students sprung it on me quite late on that she was a regular voiceover on Pepper Pig. I was delighted for her and and not at all worried she had kept it quiet all that time. As a teacher I wouldn't want to know anything about your son's professional work (or anything else for that matter) that he didn't feel like telling me. No question about it. If he felt like saying, or if it came up, or if everyone was talking about seeing him on telly, then I would be thrilled. But don't worry you're a fraud, because you're not. Master P's 'reasonable reply' to you is totally understandable.