As DisneyWorldDad said, hitting the outside of Box-o-Bot's left glove is worth 100,000 points. If you have a good gun (and good aim) you can hit it 7 times and be well on your way to a 999,999 point game.
As soon as you begin passing him, turn and begin shooting at that target. Keep steering slightly to keep him in front of you and keep firing away at him. Don't stop until he is absolutely out of sight. You may find that you can hit it more than 7 times, but it seems that's the best I've done - with a score of around 750,000 after I was finished with him.