Meaning of "he does not do so"

For Melville it is the early moment in which he reveals the internal battle that bubbles within this rather ordinary businessman. It is equally important that the narrator nearly loses his temper and that he does not do so.

^Source

Can you explain to me what is the exact meaning of the phrase "he does not do so in" the context. It seems to me that it confirms the preceding statement but it seems to be a little bit redundant (he does not do so = he does nearly loses his temper).

P.S. He nearly loses his temper. Does it mean that the narrator only rarely loses his temper?