They will ask you why you want to end your sons claim, it stops automatically when he turns 16, he then would have to apply for PIP.
They will ask you when there was a change of circumstances as there could be an overpayment, DWP makes it clear to their claimants that they have a responsibility to provide accurate and timely information about their claims, so if there was a change of circumstances a while ago then you may be hit with a fixed penalty charge, be careful with what you say and always ask for the name of the person you speak to on the phone.
