If he had started complaining in the first 30 days then i think he might have had been on to something. Its been over 30 days they probably dont need the car anymore and now wanting you to get rid of it. Tell him to go to trading standards if hes so desperate. He should also share how much mileage its done. Get everything in writing and if hes failing to comply with the mileage then hes just trying to get his money back.