Ten years old? Isn't that a little young for middle school? (Mine went to middle school around twelve.)
As for too much keyboard, that's a very difficult decision. It was quite common to buy a piano (which generally costs more than $800) when a child wished to learn to play. So I'd have no problem with spending $800 for a keyboard, assuming that he's actually serious about it. (Or that you want to play, and are using you son as an excuse. Either is good.) In fact, since the Pro X has a full piano keyboard and weighted keys, it's preparing him for piano, too. Which is even better. Who knows where he's going to end up.
But the cost of the keyboard isn't the only thing you'll need to budget for. The Pro X, like most "real" keyboards, doesn't have legs or speakers. So you'll need to budget for something to hold it up in the air, and some way for it to be heard. (Get headphones, too. I grew up listening to my father practice ten hours a day. It gets old, fast.) For the keyboard stand, don't just put it on a table. A piano keyboard is generally 29" (+/-) above the floor, which is lower than most tables. (That's at the top of the white keys.) You'll probably want a specialized keyboard stand, but they're not very expensive. Sound, on the other hand, can get expensive. Keyboards have a wide frequency range, so buying a guitar amp, bass amp, or something similar, is a bad idea.
What I don't know is whether that's a fair price for the Pro X. Someone who's got their finger on the market might be able to give a better answer.
Hope this little bit helps. You can always ask for clarifications if I'm being too cryptic.
