I'm coming into this discussion late, but maybe I can offer an opinion based on my own experiences in hockey for the last 25 years.
What you described in the first post seems to fit me well. I've always had decent skill in hockey, but it really only comes out in low-pressure situations (drop-in games with buddies) or practices. Now approaching 40 in men's league, I still suffer from the same issues I had when I was playing in high school. In a game situation I panic with the puck and play the entire time with tense muscles. I can be firing great shots in warm ups, making escape maneuvers in the corner, stickhandling through a bunch of pucks, but when pressure is on I fall apart.
For me it all comes down to anxiety. I'm afraid of looking stupid. I mentally feel like I'm inadequate, so it forces me to be inadequate. I can be coming down on a 1 v 1 with a mediocre defender on the other team, and I get so much in my own head about what move to make, I fail to really make any move, then lose the puck. As a defender, I can move laterally on the blue line with the puck great in warmup. In a game, I flub the puck just seeing someone coming my way, and then it's a 2 v 1 or breakaway the other direction.
Somehow you need to help this player overcome his anxiety and fear of screwing up in front of others. I have a strong feeling that is what this is. It's hard to identify because the player doesn't want to admit to the anxiety. It's taken me 25-plus years to finally admit my anxiety and work to overcome it. Which isn't easy to do when you're about to turn 40 and play against a bunch of 20-somethings.