I'd say start by just making one of those first missions as a peace preserver, ppl can ignore/treat him like a kid
Good idea. Something like SeeD training in FF8. You can pass here some knowledge about world, organisations, countries, more important people etc. Maybe even going step earlier at the events when he's forced to join, maybe he was forced to join by his parents.
Add a bit dramatism and danger and you have intro in Hitchcockesque style. You could also add some hard times in the organisation, expand the treatment a bit farther than just a kid. Make him a pushover, introvertic, who's anger is building from inside from mission to mission and when it explodes, some tragedy/accident/betrayal occurs which forces him to desert or normally leave them.
You can make him either a victim or a cause of something bad. Being a cause gives more tragic opportunities, haunting ghosts of the past, maybe some subconscious actions pushing him to self-redemption, making him introvertic, or even more introvertic, untrustfull etc.
In the scene with a girl a choice would be good. To help or not. The linearity is for you to decide, you could change the events a bit or in Suikoden style, after denial you could force the hero into the fight which could later alter just few dialogue sentences and run the story normally. You could add character among gangsters which could be later recruited, start a storyline concerning the cause of the assault or why he left the organisation. Maybe they didn't forget about him and want him back. What if there's something special and misterious about him, etc, etc.
Possibilities are endless.