Learning to sign can be a huge deal for kids with Ds because they often develop verbal communication later, sometimes much later, than many other kids. Sign language can fill in the gaps until they learn to create all the complex sounds associated with speech. Some kids can develop huge vocabularies in sign language - and as they start learning to say the words verbally, the signs will slowly drop off and verbal communication takes over.
So, we've been working forever, seemingly with no results. Until today. It still took some prompting, but he did do the sign without physical assistance - I'd be willing to be that if we really celebrate and reinforce it, he'll start using it of his own accord in no time. And that will hopefully be the beginning of a snowball effect. Hooray!!