Before it’s a romantic thing, Peter takes Sonny out to dinner sometimes. From there it evolves a little- Peter kissing Sonny’s forehead when he drops him off at home, taking care of him if he gets hurt in a fight, things like that. By the time Peter realizes he’s falling for him, Sonny has had a little bit of a crush on him for quite some time and is over the moon when Peter asks to take him on a proper date.
Love love love!