blake....ring 0870 062 1894 and give them the supporter number on the card and see if the seats are still available.
If they say they are, he's bullshitting you. Tell them someone has your details and have threatened to ring up pretending to be you before the derby and cancel it so you cant get in and if they do, please don't allow them to cancel it.
If the card has been cancelled, ask if it's been cancelled by the direct debit company? If it has, you know the lads telling the truth. If they turn round and say the supporter has cancelled it (he's bullshitting you), (why would ANYONE cancel their seasoncard?) then say ''well then, im the supporter and the person who threatened to do so has obviously done it.'' tell them to print you a ticket out and you'll pick it up (you cant do this on derby day so would have to do it beforehand) you dont need i.d, just your card (might have to pay £5).
But yea, ring up and see if the seats are still available, if they are, tell him to fuck off! then if he tries cancelling it, he ticket line wont let him coz you've told them he would.