Like herding cats

I’ve been assigned to manage a guy who’s got a history of being disorganized and of taking longer than expected (read: billing lots of hours) to complete tasks. Let me emphasize that I do not think he’s “padding” his hours, just has a hard time getting focused and tends to wander. This man is extremely bright and personable and technically excellent and I like working with him –but it’s been made clear to me that I’m in charge of reining him in and making sure he sticks to allocated hours and just does what’s asked for (he tends to go above and beyond, but there’s no time/budget for extras on this project). It’s only been a week and already he’s done some work that’s outside the scope. Advice on keeping him on track?