We've purchased mindstorms and musical instruments for our kids, and we've considered them *all* to be birthday/holiday gifts

If he's participating in weekly high school workshops, does he really need a mindstorms kit at home? If you've got the budget for it and he's really into it, I'd get it - our ds loved having a mindstorms kit at home when he was around 7-8 years old because he could program things with it at home that he wasn't able to do within the constraints of competitions/work/etc in the Lego Robotics teams he participated in. He stopped working with it after maybe 1-2 years - and moved on to arduinos. If you are sinking the $ into mindstorms, I'd only do it if you're comfortable with the idea that he might tire of it in a few years.
Best wishes,
polarbear