First, examine the situations. Sodapop loves his horse Mickey Mouse, and Ponyboy says that they were alike (Soda and the horse). They were both michievious and full of energy. A song about this type of personality would work, as would a song about strong bonds between friends. Also, Soda loses his horse, so a song about separation would work.
Johnny is a troubled kid who suffers for things that aren't his fault and who dies after being a hero. Songs about abuse, about dying young, and about being misunderstood would work.
Sodapop and Mickey:
You've Got a Friend; Goodbye My Friend; I Will Remember You; Lean On Me; Live Like Horses; Wildfire; Four Legged Friend
Only the Good Die Young; Goodbye My Friend; Angel; How Can I Help You Say Goodbye; Oh Very Young; The Crossroads