I have mixed feelings about this. On one hand, if he isn't verbal yet, being able to say whether he likes or doesn't like something could be useful. On the other hand, I'm uncomfortable with their reason for teaching him this. It should be a communicative tool, not a trained trick. Also, if he hasn't started yet, he'll be saying, "no" constantly in a little bit. Once they've trained him to do the thumbs down, how will the respond to his constant NOs? It sounds like they like the idea of having him do a thumbs down when they think it's funny, but they might not have much patience when he displays displeasure himself. That sets him up in some weird mind game place.