By Diane Carman Dave Stalls is a burly guy, the size of a linebacker. He isn’t really a linebacker, though. He is a former NFL defensive tackle and defensive end. Since he retired his cleats, the affable president and CEO of Big Brothers/Big Sisters of Colorado has spent 17 years working with young people. Despite his tough exterior, Stalls admits he’s “really scared,” both for himself and for young athletes. “All of us (former NFL players) are used to dealing with the knees, bad shoulders and the back, but to have your brain threatened, that’s really scary,” he said. Stalls…