Sullivan, who was 21, was a film student at the London Screen Academy.
He was found with knife wounds near the viewing point at the north London beauty spot on Tuesday.
His father, Chris Sullivan, 65, told BBC London he had lost his only son, who was referred to by friends and family as Fin.
“All of his friends said he was the light, the leader. His friends’ parents loved him. This is why it’s so tragic,” he said.