Christmas event son seemingly disinterested

[decisionsinsandouts]

I need advice on this. My son's year group performed a lovely advent service in the local church. The music teacher and class teachers had clearly put a lot of energy into it and it was impressive.
My son, who has a lovely voice and is reasonably outgoing barely opened his mouth. His self-confidence is quite shaky and I told him it was a lovely service and he had done well. He is astute and in tune with his feelings and told me he hadn't done well. I talked to him about how scary most people find performing in public and the importance of doing his best.
This is not an isolated incident. He seems to behave in this manner in group sports and class assemblies.
It is a bit of a vicious circle because when he does take a risk and outwardly put energy into some of these experiences he is proud of himself. Has anyone had similar behaviour with their child and if there are any teachers who have insight on this I would appreciate their input.

How old is he?

My ds is 12yo. He loves performing. He enjoys the rehearing, the camaraderie, the joining together. He chooses to do a theatre group for hours after school, and would up it if he had time.

But he's not a front of the stage child. He has taken big parts, but he doesn't glow with the excitement of being centre stage. He's can be as happy back row of the chorus.
He'll often choose to stand slightly behind others, while others are pushing forwards.
If he sees me looking at him, his head goes down. And if you were watching, you might well think he's not enjoying it and think "pushy parent pushing him on the stage when he doesn't want to be there."
He does love it though-just doesn't let his face tell you that.

My girls look ecstatic to be on stage and always have.

He is 10.

He was probably embarrassed. It’s normal. Some kids just get overwhelmed by the occasion.