My thanks to GordonC and coalport for their thoughtful responses. I've also had the opportunity to correspond with Thomas Grillo and Don Burns, both being exceedingly helpful. Collectively you've saved me hours of Googling, while giving excellent guidance in the decision-making process.
I wasn't aware, for example, of the tonal differences between manufacturers; and the suggestion of considering the instruments of preferred Theremin musicians makes complete sense.
My responses to questions in-kind:
GordonC
"Question. What skills do you hope your son will acquire from learning the theremin?"
Skill development is not so much the goal as using the Theremin as a device for furthering his general interest in music. My son, Robby, is already acquainted with the Classics, and is interested in revisiting them with the addition of a Theremin.
Robby also studied electronics last year, so the Theremin seems a practical, artistic extension of those studies.
coalport
"It seems like an odd choice of instrument for a parent to choose for a teenager. May I ask why you have chosen the theremin?"
Robby's been exposed to a year of drum and then keyboard. Despite initial enthusiasm his interest was short-lived. Violin was our next choice, until he heard what he thought was a violin but was actually a Theremin.
"Has your son shown an interest in the instrument?"
Yes. The more we've looked into Theremins, the more his interest has grown.
"What sort of music does your son like?"
Classical (he's a 4-year student of Ballet), Techno, Surf and Heavy Metal. Actually, reverse that order and you'll have his preference (for better or worse!)
"Is he a naturally disciplined and fairly obedient kind of kid, or is he more of a "Dad, willya get outta my face" sort of young man?"
He's naturally disciplined; we've a terrific relationship (perhaps an affect of my wife and I being "older" parents, 50+).
"Has he had any experience with any other instrument? I ask this because most musicians agree with the late thereminist Clara Rockmore's statement that the theremin should never be someone's first instrument since you can't learn music on it."
Robby was tutored both on drums and later keyboard. I would counter that if Robby's interest in music is furthered due to the Theremin -- if his creativity is expanded by "re-mixing" the Classics, cinematic scores and even Rock -- and if the Theremin causes him to develop a better "ear" for music, then the Theremin would indeed have succeeded where traditional instruments failed. As for reading music, there are abundant texts on the subject. And as for composing, I'm not certain Robby would aspire to that on any instrument. At least not yet. However, as I myself am not an accomplished musician it's likely I don't understand or appreciate why someone could not learn music with a Theremin. To that point I must yield to the opinions of Ms. Rockmore and others.
Many thanks once again for the thoughtful advice and excellent questions. Your responses will benefit not only my immediate family but our network of like-minded home schooling families.