Maybe this can help a bit.
Right now your son thinks that the test is the enemy. It isn't. It's his anxiety that is the enemy. He doesn't have to compare to any other student in the class. He doesn't have to be Superman.
He just has to pass the test. Once he goes into the testing room, his preparation is done, and there's nothing else but to do the best he can on the silly thing and be done with it.
If he's looking at the test and saying to himself, "Wow, this is hard," then he's not thinking about the test. If he's looking at it and thinking, "I wish I weren't in this room," he's not thinking about the test. If he's thinking, "I don't know if I know the answer," he's not thinking about the test. In all those cases, he's thinking about himself instead.
He's thinking about the test - and that's where his mind should be - if he's thinking, "Oh. Question two. Well, I know *that* choice is not the right answer. I'll take a look at the other choices."
He should not be thinking about what will happen next week or what anybody may think of him or what time it is. If he finds he is, he needs to get his brain back to where his body is.
Encourage him to get into the world of the test questions, eliminate the answers that don't work, and find the best answer possible. If he doesn't have to do the questions in a set order, he could look down the page first; he'll find some he can answer confidently right away. Then he can start dealing with the other ones. All he can do is all he can do; that's the way it is with any of us when we're testing.
This is what I had to do in college and in high school. If that test was "just something to do," I did much better than when I wondered what awful thing would happen to me if I failed. If I did fail, once I found out it hadn't actually killed me I could pick myself up and go talk to the teacher about how I could do better. I still do that.
I don't know anything about your son's sort of study or how often people can test for a license. I do know that in some fields people may, and often have to, test more than once. My husband is a CPA and the exam is unbelievably difficult. He took it once, didn't pass, said, "All right - now I know what to study more," and took it again.