Op, I really do understand your sadness at you little boy not being chosen. You feel that his confidence is at rock bottom and of course that feels worrying for you.
Can we ask what age (or key stage, if you want to be less specific) he is at please?
When I read that he hurt himself and badly scuffed his knees, but seemed unable to tell his teachers, I could very much see how confidence is a real issue for him. The confidence and ability to seek comfort from key adults is something we would want/expect to see develop during the Early Years.
All children are unique and non of us here know your son, only you do, but some of us will have many years in working with children and would like to support you in supporting him.
One of the things that stands out from your posts is a feeling/worry maybe you have, of “I want to tell the teachers, that this feels unfair to my son, but I can’t/shouldn’t.” This worry seems to mirror the response of your little boy when he fell and hurt himself “I have hurt myself badly, I can’t/shouldn’t tell my teachers”.
If you Google “construct theory” it may help to explain a possible reason for this.
Very basically, as we develop and grow we develop our blueprint for behaviour and values by closely observing those closest to us. It is the reason some people are never late/ some people are perfectionists/ some people keep choosing abusive relationships/ some people always take their shoes off going into another persons house. Constructs often have a “judgement” feeling attached to them.
From the example of what I perceived to be mirrored behaviour, above, the construct would be “It isn’t good to share feelings with…”. In some families it might be “…anyone” in some it may be “…people outside the family” or possibly ”…someone in authority.”
By considering construct theory you can also see how the confident PTA mum’s child is maybe more likely to be given roles where confidence is key. Construct “putting yourself out there is good.” We can also see how some of us attach value to central and key roles “I must be first/win”.
There used to be a saying “the apple doesn’t fall too far from the tree.” But who says the tree needs to stay on the same projectors? By carefully selecting the right growing conditions, and what it is exposed to, the little tree might end up even lovelier than those that went before it.
If any of this resonated, my first advice, before anything else, would be to work on that construct which is holding you both back from expressing how you feel to the teachers. Allow him to see you express your needs/wants/desires and you be ok before/during and after, and keep doing it. It will start to give him the confidence to do the same. He will be gaining a skill that will support his happiness throughout his life, and you might find some positive benefits for you too.
I hope that helps. And good luck with the show. I am sure whatever role he has, you will love him in, because he is your boy.