Did his game change at all after breaking his jaw? He had 9 goals his first 13 games in the league. Only 16 goals in the 72 games since. Or was the fast start because the league didn't know him yet.
5v5 stats:
First 13 games
23.5% shooting percentage
11.2 shots/60
4.61 iHDCF/60
1.39 ixG/60 vs 2.62 G/60
Last 72 games
7.8% shooting percentage
7.6 shots/60
2.68 iHDCF/60
0.84 ixG/60 vs 0.59 G/60
Long story short, he was getting a lot more shots and high quality chances in those first 13 games, and was getting really good puck luck to go along with it. Then something happened, either fatigue, or the fact that teams and goaltenders adjusted. His shot rate, high danger chance rate, and ixG rate all plummeted - the eye test suggests he's having a really hard time getting to the middle of the ice, and teams have been able to keep him to the perimeter without too much trouble. His shooting luck regressed as well, which compounded those factors.
The data seems to suggest he's shooting a lot more from the perimeter than he was at the start of his career, though both his shooting attempt volume and expected goals have dropped dramatically over the course of his career - definitely not trending in the direction you'd expect for a player of his pedigree. The graph below is his career rolling CF/60 and ixG/60 numbers at 5v5:
Regardless, this is starting to look less and less like just a cold streak, and increasingly seems like he needs to make some serious adjustments.