Try this:
Get a handful of yummy treats and have them in your pocket. Get a can of some sort and put 10-20 pennies inside. Get ready. When your dog barks, shake the can immediately. That should startle him enough and get his attention. Then tell him to sit, and when he does, give him a treat immediately. Be consistent. Eventually you should be able to tell him to sit--or down, if you prefer--after the first warning bark.