If he barks quite a lot/regularly, you might not even need a clicker to teach it on cue.
While he barks, say excitedly "Speak! Speak!" or whatever word you choose. If you can accurately predict exactly WHEN he's about to bark, give the word a split second before, and keep repeating it during the barking session. Essentially, you're telling him what he's doing, and pairing the behaviour with a word.
Do this enough, and before too long, when you say "Speak!" excitedly, he'll start gobbing off automatically :)
Then you can give him a treat and introduce the "Quiet" or "Sssh" cue. Because he's not actually barking AT anything this time, other than your "Speak" cue, he should shut up while he eats his treat. When he looks at you for another treat, say "Quiet" (telling him what he's doing) - give treat. Repeat a few times.
Now you've got two cues - "Speak" means let's have a jolly bark. "Quiet" means shut up, there's a treat on the way.
When you're fairly sure he's got the idea, practise asking him to speak, then asking for quiet, and treating. Then speak, then quiet, etc, so he gets a clear idea of both, opposing cues.
If he's a barker anyway, you probably won't have to treat for barking - he obviously finds it rewarding enough in itself.