One night it seemed as if all the neighbor kids decided to be outside, all at once. James hadn't met any of them (they are in the newly constructed houses) and so he was naturally shy and hanging back. I tried to encourage him to go outside too but no.
One of the Dads was outside too and I told Jim that if he went over and said hello, James would follow and then fall into playing naturally. Yup!
And off they go!
And he's been playing outside ever since! It's been the Summer of Nerf!