Jurist gezocht: na aankoop appartement worden we geconfronteerd met een huurder

Toelichting
We need an advice to understand if we have any chance to winn this issue.
I have bought an apartment in may 2018 where in the b/sale contract the seller warrant that at the time of actual transfer the immovabol property is not subject in right of any use and is vacant and clear, with the exeption that the new owner will make rental contract with his son from summer 2018 untill 2019.
But now the tenand clame that he used to have a permanet rental contract before the apartment was sold and he does not want to move out.
I need a letter from a lawer to be send to his father (the old owner)