It's a little after season, but I used the local Little League fields when introducing Koa to more kids when he was younger - maybe find something similar... soccer, football, etc and start walking by at a distance so he can get used to the sight, movement, etc of kids. I read something on here before I got Koa that I always tried to keep in mind - to us, people are people, but to the dogs kids aren't the same as adults so it's like introducing them to something completely new. Kids talk different, act different, scream, run around, etc.
Definitely take him to as many places as you can. Have you enrolled in any obedience classes with him? A friend of mine had a GSD who had a similar temperament and when they enrolled her in an obedience class with different ppl, dogs, etc she became more confident and I've really noticed a change in her.