The theme of A Separate Peace is Search for Identity
When Gene talks to Finny on the phone is an example of the search for identity
"'Listen, pal, if I can't play sports, you're going to play them for me,' and I lost part of myself to him then, and a soaring sense of freedom revealed that this must have been my purpose from the first: to become a part of Phineas."
This is a search for identity because Gene believes it is better to be Finny than himself. He became excited about being 'part' of Finny.
The next scene takes place at Finny's funeral. This quite obviously fits the theme.
This shows that Gene believed some of him died with Finny. That he himself in some way is a part of him
"I could not escape a feeling that this was my own funeral, and you do not cry in that case."
The last example I am going to use is from Chapter 11 page 156 of A Separate Peace.
"Over my cot I had long ago taped pictures which together amounted to a bareface lie about my background-weepingly romantic views of plantation mansions, moss-hung trees by moonlight, lazy roads winding dustily past the cabins of the Negroes." - John Knowles
The first quote says that this is what Gene wants to be and what he wants people to see in him.
He continues, "But by now I no longer needed this vivid false identity; now I was acquiring, I felt, a sense of my own real authority and worth, I had many new experiences and I was growing up."
The second shows that Gene has found a sense of identity. He feels he has worth and that now he has something to offer others.