Attached is a .zip file of a song with three quickly made Theremin sounds:
Select track 1 to hear an EXS-24 version (no samples needed!)
Select track 2 to hear an ES2 version, slightly brighter than the EXS version
Select track 3 to hear a mellower version of the ES2 sound
You'll probably want to adjust the glide rate, attack, release, vibrato rate, etc. to get exactly the sound you want. Note that all of these produce vibrato automatically. If you want to use the modwheel to introduce the vibrato you'll just have to do a little extra programming to get that to work.
All totally modifiable, but perhaps enough to get you started. (Oh, and pardon the cheezy reverb -- it's just a Space Designer instanciated on an Aux and no actual reverb selected).
Basic construction of the Theremin sound: a sine, triangle, or square wave (low pass filtered) to give you a predominantly pure tone. Glide (portamento) and vibrato are the chief pitch characteristics. Slightly slow attack, and most importantly, monophonic operation (one note at a time). Play legato. For some reason the EXS version glitches every once in a while even though it's set to a legato monophonic mode. Still, if you can deal with that, the "feel" of the glide on the EXS is particularly good for Theremin imitations.
I know you want Beach Boys, but for a more "sci-fi" sound from the ES2 versions, add distortion or filter drive, and/or select a brighter waveform (like square) but don't close the filter down on it quite so much -- just enough to give the sound a little edge. There are a million ways to get variations you can give to the tried-and-true pure-tone Theremin sound.