This is not normal for a teenager boy, hes only a couple years younger then me, however no guy i know will ever do that, some people get the motive by watching porn, take away his computer/laptop for a week or so, and dont let him play on his console i.e. xbox, tell him this ur punishment for what u did, dont let this one go off easily otherwise hes more likely to do it again, cuz he got away easily the first time, block the porn websites off the laptop/computer, put a proxy on ur computer, if u got sky digital type ur pin in on the settings option and disable the 900 porn channels. btw eres a link below on how to block porn, ect.. and yes its porn that temps ur kid to do this, im actually angry how someone can actually do that to their mom, he shud kno better.
http://www.ehow.com/how_4466699_block-porn-completely-computer.html
ur sons likely to have porn stashed away i.e. magazines, on his hdd, pictures and vidz, look for it delete it, get rid of it, and then block the porn websites, and apply the proxy, get a computer/laptop monitor to monitor activity on the computer, u can also warn ur son, uve put a monitor software on his computer therefore if he does anything hes not supposed to be doing on the computer, u will take away his privilege to use the computer.
http://www.microsoft.com/en-us/download/details.aspx?displaylang=en&id=4865
http://download.cnet.com/windows/monitoring-software/