Is it possible to replace this quick release part on a Motiv Rockpoint? My son has been using this bike, and the other day he came home with a bolt that had come off. I checked it out, and it looks like there's also another part missing—the other end of the quick wheel release. I assume it's the same as this one on the front. 

Is there any way I can get a replacement? I think it's maybe just called a "quick release nut", but are they at all standard? It doesn't look like this bike is even sold anymore, so could I just buy a generic one (or even the whole quick release assembly if necessary)?
Quick releases are standard parts. You can replace the whole unit with a new one. Normally you buy them in pairs, front and rear together.

All you need to know is whether it is for a mountain bike or a road bike since the size of the rear one varies due to different rear axle lengths.

What is more worrying is the fact that the nut came off first place. This can only happen if the QR isn't properly fitted which is a potentially dangerous and even lethal situation. Check this video: