What are the default controls for Chop on the PC? As in, how to take him for a walk, etc.
I guess you must have been progressed the story to a certain degree.
It usually shows the controls in the top left corner. For me it was (I suppose you must be unarmed):
Hold Right Mouse to activate the further Chop-menu.
Here is the menu I got while holding the Right Mouse button. Notice, that there is no option for playing fetch. I suppose this depends on the environment you're in and if you have picked up a toy or not.
I suppose you know this but just for completeness: Make sure you're playing as character "Franklin".