It's not like you just need the cadastral documents to get everything torn down right away, unfortunately it's a bit more complicated than that. First try sending him a registered letter explaining the problem and asking for clarification - maybe he didn't even realize he was encroaching or he can fix it easily; if he doesn't respond or refuses, it's worth contacting a surveyor or lawyer to do a proper verification on the ground, because sometimes cadastral records have errors or different interpretations. In the meantime document everything with photos and measurements, and if you can't reach an agreement then yeah, you'll need to think about legal action, but at least you'll have tried the peaceful route first.