Steven has come to terms with his mortality, accepted responsibility for his existence, and knows that he determines what happens in his life. Existentialists would say that Steven is .

Existentialists would say that Steven is attaining authentic existence.

Explanation:

Existentialism is a philosophical theory which views the individual's existence as free and views him/her as able of determining his own will and choices throughout life.

Existential authenticity is the state of being, of taking charge of one's own emotions, relationships, sensations, etc. It is when an individual knows his/her being and is able to choose through free will.  

In this case, Steven is attaining this because he has come to terms with his morality and he has accepted his responsibilities for his existence, as well as understanding that he is able to determine what happens in his life.