Version 10.2 fixed a problem that prevented players from completing Paul and David's story if they had “purchased” Paul beforehand.
Which version are you playing?
If it is earlier than that, try downloading the latest version. If you already have version 10.2, I will have to check if there is another problem.
Remember that for the fix to work, you need to load a save file from before you “purchased” Paul. Save files in which the player has already advanced in his story before updating the game will not work.