Unless you're a trader he hasn't got a leg to stand on. He's just been unlucky here and he hasn't been reasonable when you've offered to help.

I sold my wife's old car a few years ago and the new owner called me a few days later to say it'd blown up and he was bringing it back and he wanted a refund I told him this isn't Tesco's!!!! In the end he did accept £200 towards repairing the engine just to get him off my back and I've never heard from him again.