To the op - your son needs a license. At my uni they give the inspectors free reign to enter the halls and bedrooms, and a few weeks ago about ten people got caught. You are allowed to deny them entry but obviously that's no good if you're out at a class. However, you can get a refund for any months that you don't use, but it's a long winded process.
This is a bit off topic, but I'm just curious about something I noticed last year when I was helping a friend (an international student) sort out her TV license. Basically, she set up a direct debit and for the first six months she was charged double. Obviously she was worried that she'd been overcharged, and as her english wasn't great, and she asked me to deal with the licensing people on the phone. It turns out that if you pay by direct debit then they take double for the first six months so that you're always six months in credit with them. Is this just me or does this seem a little unfair, especially for students/internationals who might actually only need a license for a year? My friend wasn't able to get a refund for this extra money either. I just wondered if anyone else had experienced this ...