Buyer wants to cancel a purchase contract one month after purchase?

Good noon dear forum, the situation is following, about a month ago I sold a vehicle privately for about 10 thousand euros. The vehicle ran flawlessly, had no problems and all maintenance work were always done. The buyer came to the inspection and a few days later he bought the vehicle then after a little negotiation on the phone and picked it up. Possibly still to be added, that the buyer planned a few days later several thousand kilometers in the Urla ub. So far so good, but now a letter from a lawyer who is to act as a “mediator” and who was commissioned by the legal protection insurance behind Mr. xxx. came with the approximate words: “Mr. xxx. demands that the purchase contract be reversed,ggf with judicial help, since the vehicle has a defective in the cooling system, of which they knew in view of the high coolant consumption! and they did not point him to this deficiency. that they got in touch with him! ” And this is simply not true, the vehicle had no problems with the coolant or anything else as long as it was in my possession, it ran perfectly and was used until the last day and the buyer of course also made a test drive and looked at everything before buying. My question to the forum is now, what do you think is coming to me? and what would you recommend to me in this situation? many thanks in advance lg