As someone who was a troublesome teenager not that long ago... let me offer some suggestions. I would suggest a bios password.. This password is required to boot the computer. (and the option for safemode can only be accessed after entering the password) To set this up (if possible) depends on what type of computer you have so the directions and keys to use to get to the bios at the following link may not work for you exactly as specified but should be similar... if you have trouble accessing the bios try googling your model of computer and bios and you should get some help.http://www.lockdown.co.uk/?pg=biospsw
If you have any questions about this post back with what kind of computer you have and I'll see if I can get you specific instructions if your bios is capable of password protection.
The other option is just always keep the power cord ( or the battery if it's a laptop) with you so he can't power it on