Conversation so far:
What did Stephen Sondheim do in his early years?
He attended the New York Military Academy and George School, a private Quaker preparatory school in Bucks County, Pennsylvania where he wrote his first musical, By George.
When did Stephen Sondheim graduate?
1950
Did Stephen Sondheim join the military?
No, he did attend the New York Military Academy, a college prep, co-ed boarding school.
Did Stephen Sondheim's mother abuse him?
Sondheim detested his mother, who was said to be psychologically abusive and projected her anger from her failed marriage on her son:

Question: Where was his father?
Self-contained rewrite: Where was Stephen Sondheim's father?