Some inconsistency is to be expected in young players. I think that's what we've seen in previous years with Quinn.
This year seems different. It's not inconsistency, it's complete collapse. He's not just failing to convert his chances, he's getting beat consistently in all aspects of the game. He seems slow, no explosiveness at all, quick to tire, his hands are fumbly, poor decision making, not strong on pucks, not reading plays well, out of position.
My guess is that he didn't have a good summer of training, maybe due to laziness, or illness, or personal issues. Wasn't prepared to start the season and he knew it, so his confidence was shaken.
If it were me, I'd give him a good chunk of time off (2 weeks, a month) and have him really work on his conditioning, strength, and puck skills. Have him work towards a return date when he can come back and be prepared to play his game.
Some would say we can't afford to do this, but I think he needs a reset or he's going to struggle all year.