My son was like this — very skilled, but nervous. The answer was small group reps. He got into a small training group with bigger, older kids who played very physically. Playing 3 on 3 with big, aggressive kids totally cured him of playing hesitantly because there is nowhere to hide in 3 on 3, and it totally changed how his coaches perceived him. Having an aggressive attitude is a big part of basketball, and - within reason - makes other skills like shooting better , so if you can find small group training like that or an AAU team that specifically trains aggressiveness, it will help a lot.

OP every athlete plays tight at times, even the pros.

He's getting external validity. Now he just needs more playing and practice time (kind of like exposure therapy) so that it's hum drum and not an event.

Some athletes also use compartmentalizing and cbt to compartmentalize. Lots of envsioning techniques.

Just help him keep perspective so he stays calm. This is NBD. Nobody will remember this game in a few months. Think of the cosmos.

My freshman is a savvy player but holds himself stiffly when he's nervous or feels self-conscious. If he doesn't get as much playing time, this feeds into it, so when he does get on the court, it's clear he's holding back, self-consciously, afraid of messing up and getting more physical.
His issue isn't lack of ability, per his coach: It's lack of physicality.
The problem is, it's a mindset thing. He isn't sure how to switch that physicality "on" when he's anxious and cognizant of his own performance.
Any advice? He received a very supportive email from his coach.