My son does the same thing - he doesn't run when outside. It turned out that he'd get really chill when in the park with his class. He liked to wander, would listen to the teachers when they'd remind him that he was starting to stray a bit far. Traditional logic says "he needs to run to get his energy out", but energy wasn't the problem. In the park, his sensory needs were met. There was no booming echo from the shrieking voices in the school. No shelves of brightly colored bins full of toys lighting him up to come grab them.