I'll go along with Nikosony. We all experience bad payers and it's often emotionally difficult to deal with. My particular method is that I only allow one open invoice. In other words I don't work for a company once they owe me more than one bill.

I would warn your "mentor" that you expect to get paid and tell him well before the day of the gig that you need the money up front. If he doesn't pay, then you go home. Remember the old adage "Only a fool will work for nothing and many a fool does."
There's a difference between "learning" and "working". If, when you're filming, you've got somebody supervising you and giving you hints and tips... that's learning. If you're supplying video for him to earn money... that's working and you should be paid for it.
After four "freebies" you should be getting paid, unless they were poor and he suggests that you need more experience.