I wasn't trying to be judgmental mancmo, I don't know if I could continue under such circumstances (I didn't see the programme and am fairly ignorant re Tourettes so it is difficult to be sure how I would react).
As regards how you should respond, I guess he's part-expecting rejection as it's probably often happened before, so the best approach would be an apologetic explanation of your feelings which as you quite rightly say you can't help. And wish him better luck for the future.