Would Rex be considered successful?
Many dictionary definitions of "success" mention the accumulation of wealth in the form of money, property, and other tangible assets. Other looser definitions of "success" mention ideas like personal fulfillment, doing one's best or even simply living according to one's own values. Though Rex is unlikely to be described as successful in terms of wealth and assets, he may fit one of the looser definitions of success mentioned above.
Rex has always lived according to his own standards, and he refuses to play by the seemingly arbitrary rules of society. Though his choices, from an outsider's point of view, might seem selfish, they are all Rex's choices, and he might say that his choice to be independent is a definition of success in it and of itself. Rex finds living within the confines of everyone else's expectations suffocating, and he challenges these expectations with determination. He chooses to live by his own rules, and he definitely succeeds in this way.
Success is not an easy word to define, and it is not an idea that exists in black and white terms. Rex's kind of success lives firmly in the gray area, where it can be argued and debated. This is only one way to interpret Rex as a successful person.