Squirting him with water is one way- though that would likely be one more thing to bark at. Spray bottle doesn't work n my dogs- they catch it in their mouth- think it is a game. Teach him to speak on command would be a better idea. Once he knows the command to speak, you can get him to understand 'no speak'- or, as our Natalie knows the command- 'mime dog' and she changes her bark to a whisper. This was the only thing that helped us. When I talked to the vet about it- he wanted to do surgery on her vocal chords. I definitely wasn't keen on going that route.