he'll turn up eventually and hopefully pay the price for his disappearing act, literally.
Were the bills in his sole name or were others involved?
As Mrs Pegasus said, return all post unopened. If it is getting a serious problem you could try writing a standard letter to whom it may concen to send back with them stating that the addressee has not been resident at that address since x date and that his whereabouts are unknown.
If you know a phone number or another contact address for him such as his parents' place you could put that.
Say you are sorry that you cannot help them further but to ask them to amend their records and inform all othe related parties such as debt recovery agencies and legal advisors.
You could also give this to any bailiffs that call too.
I've had similar problems with bailiffs after previous occupiers at my places and it's crap :(