If you were to say “he has courage I’ll never have”, that almost could be taken to mean that you will never have courage at all.
By saying that he has “a courage” it implies that you have courage too, but that his level is higher than yours.
So in effect, the sentence could be re-written to "I will never be able to achieve his level of courage.
My English department says much English, particularly spoken, is transactional. (= as long as you understand it, that’s fine). The sentence you quote also has a ring to it.