A pitcher can only do
his job. He can't play all nine positions on the field. If he is getting batters to hit grounders and flies, he has done all he can do. All anyone can ask of him. The rest is beyond his control.
A pitcher must have the mental toughness to believe and accept this.
That is not to say that there won't be some frustration with errors made behind him. But he cannot let that frustration result in an outward display, nor can he let it affect his performance.
As a parent, you can continually reinforce that your son did HIS job to the best of his ability with maximum effort, and that is all that anyone can ask of him. Tell him you are proud he did his job and didn't show any frustration. Remind him that the mental toughness he is developing will come in handy down the road. Even the best defense can make a mistake at a critical time, and they will need the pitcher to encourage them and maintain his cool as an example.
As one pitcher's dad to another, you may not want to sit close to the other parents. They may overhear you muttering underneath your breath.