He's just a baby and still unsure of himself in the big, wide, scary world. I'd be inclined to go easy on him a bit. Patience, as always, is the secret. Perhaps, while he's still little, you could carry him for the first 50 yards or so. It'll take some time for him to get his confidence. Work with him, not against (as I'm sure you are) and before you know it he'll be dragging you out of the door and up the road so fast you'll be back asking for advice about him pulling like a train...