Well, it sounds like a little revenge is in order.
1) Never let your kids (of any age) know your password
2) If you have a kid who is irresponsible (like many of us have), do not give them Administrative rights on your computer (instead give them their own cheap computer to mess up), give them a limited user profile so that they cannot do crap like this to you
3) After recovering your files (if possible), then lock his account down, and take away his access to his files for a little while and see how he likes it. If you are really vindictive then delete his files all-together. Life sucks when you have to redo your homework
As for file recovery; There are a multitude of free and for pay software titles out there to do data recovery if they are truly gone. I would give EaseUS a try. Preferably you will want something that works outside of Windows and runs on it's own bootable CD, but with free products your mileage will vary. I ahve had some good luck recovering files from corrupt drives, and deleted partitions, but when something is on the system drive and it gets deleted Windows starts re-writing that space very quickly, so the damage may be irreparable.