He’s dry heaving because he’s so nervous, but he’s facing that nervousness and taking on the challenge by going out on the field and playing. He’ll win some games and lose some. He’ll play well at times, and other times not so well.
What’s important is that he’s figuring out for himself that he can do really hard things.
If everybody in this country was asked to do something that made them nervous enough to cause them to dry heave, 98% of people would just refuse to do it. They would crumble under the weight and stress of what’s being asked. Your son is figuring out that he’s part of the 2%. That’s a ridiculously good thing.
My advice is to keep giving him positive encouragement and continue to build him.