How does Dunstan Ramsay grow individually throughout the novel Fifth Business?

Dunny does some serious growing and changing in the novel, but he also realizes his limitations.  One way in which he does not grow is in his relationship with Boy Staunton.  He always had a long-simmering contempt for him, and kept important facts about Boy's own life from him (like the fact that he still had the rock that hurt Mrs. Dempster, and that he had supported her in mental institutions for years, and at one time Boy's first wife Leola had propositioned him), all while remaining, on the surface, a friend. 

But Dunstan makes a big change in his later years, specifically in regard to Leisl.  She teaches him that love, friendship, and even a sexual relationship doesn't have to be smothering, as it was with his mother.  Dunstan spent a long time learning this; because he never had a long-term relationship (other than Diana) that worked out very well, he lived a somewhat stunted, bachelor life well into his 50s.  But knowing Leisl for many years changed him, so that in his later years he could fall into a kind of companionable relationship that didn't frighten or threaten him.

Also, Dunstan did come to terms, through Blazon's analysis of his psyche, with the figure of Mrs. Dempster in his life, especially with his feelings of guilt about her.  In a somewhat ruthlessly practical twist on the idea of God's plan, Blazon counsels Dunstan to accept that Mrs. Dempster's sanity may have been sacrificed to God for a reason, and not to dwell on it and make it his personal problem.  While Dunstan doesn't always agree with Blazon, his advice is certainly good for his self-examination.

Leisl is the one who pegs Dunstan as "fifth business" -- a cognomen which certainly doesn't always fit him -- and makes Dunstan consider him less the protagonist of every drama involving him, but possibly only a supporting character.  This can make a particularly egotistical person depressed, but for Dunstan it can be quite liberating.  Since the weight of the world (or the story) isn't always on his shoulders, it left him free to do what he liked and become more fully who he was. 

I wouldn't say that the growth of Dunstan was dramatic; it was more organic and believable as the maturing, finally, of an old man over many years.  Dunstan began life with fewer weaknesses, perhaps, than other people, but the weaknesses he did have were great and limiting.  That he mitigated these significantly during his lifetime, for his and others' benefit, means that he made real and permanent progress.