Q : If the Continnum's told you once, they've told you a thousand times. DON'T PROVOKE THE BORG!!
If Q are omnipotent / immortal / all powerful, why would Q tell his son not to provoke the Borg?
Q himself provokes humans (Janeway & Picard) quite regularly.
Do the Q fear the Borg?
Or could it perhaps be that the Q just don't want to annoy the Borg, since the Borg are possibly the most powerful guys in the galaxy (outside fluidic space) and the Q don't want to be responsible for anything the Borg do out of anger? Since the Borg could probably wipe out a good chunk of the galaxy if they felt like it.
Thanks to IQAndreas for finding a clip of the scene in question:
The question : What would be the effect of a Borg assimilating a Q? definitely explores similar territory to this question, but no answers really address that part of the question. There are a few opinions as to why Q wouldn't want his son to antagonise the Borg, but certainly no definitive answer on whether the Q fear the Borg or not.