Depending on how much overall contents he has you have a couple of options.
1 - he can take out his own contents insurance which will cover all of his possessions within the house - exactly the same as your Home insurance (obviously he only takes the contents side)
2 - If he is a student (which it sounds like he prob is) you can insurer his contents as an extension to your main contents insurance. Speak to you broker but it is a simple addition onto your policy (generally giving a limit of circa �5,000 - but can obtain higher amounts is you so wished)