I agree with Lisa - get rid of the evidence and claim you know nothing....
He must have left it at a mates house, on the bus, at school, at the gym......
The list is endless!!!
Then, after a day of frantic searching you can do your angelic mum bit and offer to buy him a new one!! He will think you are the best mum ever, and no-one gets shouted at!! A winner all round I think!!!