If he is in fact using the Theremin to control the synthesizer, it would be a great tool for a "transitioning" effect on a specific sound.
Transitioning as in, instead of pushing a different key on the keyboard, he can slowly fade out a note by using his hand or he can change notes gradually. On a keyboard, when you push a different key it's a done deal and the note changes.
As far as Joe's new sampler goes, it's a JazzMutant Lemur. Joe's version does not look like the one that is for sale at the moment. I think he either had it custom made or it's an older version.
Here is the one he has but I don't see it on their website. I imagine they are all capable of the same thing.