I'm guessing that he got hooked on Oxy due to previous injuries. I used to take it for pain, and then it was easy to take the left-over pills when something was a little sore but I really wanted to sleep without waking up. I still have a bottle somewhere just in case my knee starts to act up. Although last year (?) when I took a pill it seemed to mess up my head more than help with the pain, so I haven't taken any since.
I think the fenteynol is a contaminant in some drugs - added (unbeknownst to the user) to spice up the effects.