Hi Jack, let me just clarify your program. In your Original Post you said that your son "sometimes he just lets the DL go anywhere and it goes to the Windows 7 partition and fulls it up with junk". What exactly did you mean? From your subsequent posts I am not sure whether you meant simply "download" (i.e. photos, movies, books, etc) or "download and install" programs?
In the former case, the DeaconFrost's advice is almost certain to work, unless your son is doing it on purpose, but given that this is his computer, it's unlikely. So assuming he simply does not pay any attention to where the downloads are saved, he most likely simply clicks "download" and then "save". In this case the file goes to the default download location. What this location is, depends on the browser.
If you use Firefox, the go to Options (or Tools, depends on the FF version). Under "General" tab you can see the options for dealing with the downloads. I assume you have the check-box "always ask me where to save files" unchecked, but if not, clear it. In the field "Save files to" use the "Browse" button to set the default download location to any place you want, presumably off the system drive.
If you use IE9 (that's what I currently have, in older versions the precise steps might be different, but basically the same) you press Ctrl-J, the "View Downloads" window will appear (you can also go to options -> View Downloads). Press Options at the botton of the window and use the Browse button to set up the default download location.
If however your son not only downloads stuff, but also installs lots of programs, then I'm afraid these would go to the appropriate folder on the system drive (usually C:\Program Files). While it is possible to move this folder, I would not recommend it. Moreover, most programs that can be downloaded for free are not that big. So in this case just talk to your son.
Finally, I should mention that you made a smart decision in giving your son his own computer. At least he's not messing with your own machine, which might be important to you for work or otherwise. Now I would make a step further and try to teach him to take care of his own things. His machine should be his responsibility. You can slowly teach him how to use it properly. This will be useful for him and for your relationship as well.