The question doesn't quite make sense - if some unknown person was trying to get credit in his name, how could he disassociate himself? Presumably you don't know who it was? You haven't possible forgotten something you bought - if you buy something from a catalogue or pay for furniture or something expensive in installments this will show up on your credit file, for example.
Your credit file shows searches as well as actual credit accounts. If only searches are showing up, you can be fairly confident there are no unauthorised debts in his name, although the searches themselves can affect your credit rating. I would suggest he gets a more up to date credit search done with all of the main credit checking agencies.