BUY IT!! May I just say how worthwhile it would be (assuming he uses it, of course). I started coaching a First
Lego League team when DS was in 4th grade. He didn't currently own his own Mindstorm (he had played with a friend's), but we got him one that Christmas. He loved it and used it all the time. I was constantly astonished what he would create.
Two Christmases later, he asked for a second one (both so that he could have two and so that he could build more complicated designs). We gladly bought him a second.
He (and I) competed with FLL for 5 years, with our team ending up Regional Champions for 3 (or 4??) of those 5 years! And as a Freshman, DS is already on the HS Robotics team and excited about competing with them.
It may seem like a "toy" but if your son has a knack for engineering and/or building, this may turn out to be a life-changing gift for him. My DS is already excited about the future job of choice.... robotics engineer! And while he doesn't use his robot daily any more (he is nearly 15), he still putters around with it on the weekends and such.
In addition to the Mindstorm, check
Amazon for some books about things to build with them. (Can't remember the titles right now).... and of course, there are lots of plans on the web that he can use. Some of the things that DS has built.... a birthday candle that sings "happy birthday" and "extinguishes" when you blow on it, a crawling scorpion, a mini-guitar, an automatic cocoa stirrer. If other folks are wondering what to buy your DS, there are lots of "extras" that he will eventually want.... touch sensor, sound sensors, light sensors, etc.
My last suggestion.... do NOT buy used. These things do have a limited lifespan (just like desktop computers and such) and with the use/abuse they see from a Lego League group, it is entirely possibly it won't last for long. ..................P