I'm a freelance programmer from Belgium and I have developed an application for an American client. We agreed on 800USD for the completed project. I finished the application and met all his requirements. We only had a verbal agreement about the project and we discussed payment dates for the project. I received 4 200USD payments over the course of two months, after which I sent him the completed product.
After I sent him the finished product everything seemed good. However after some days he said that it was unusable because switching between pages was "too slow". He demanded a refund because he said that he was unable to use it. In my opinion it was fine and I didn't see what was wrong with it.
Now he's threatening me with 'legal action' if I don't refund him the full amount, but legally speaking what can he actually do. Can he even sue me, since I'm 17 and live in a different country? And is it likely that he would be successful, since we never made any form of contract?
Any help is very appreciated, thanks!