Start with getting him to speak as a trick. Hold a treat in front of him, let him sniff at it hard. Raise the treat high enough that they can't get at it. To keep their attention I always keep asking in a happy pitched voice "Do you want it?" Keep asking until they are jumping and really trying to get it. Once they are really excited, in a quiet, happy voice say speak. Say it almost as if you are barking the word speak and say it quick. This is the only command I ever repeat over and over. Just keep quietly barking speak until you get him to bark. With any other dog they usually bark, but Bailey woo wooed first. Raise your voice and say louder until you get a bark and reward and praise. It is easier to demonstrate than to say, but it works like a charm. It does not take long for them to get, if they are not barkers they actually enjoy it. Once he gets to speak on command when he needs to go outside bring him to the door have him sit and tell him to speak. As soon as he barks open the door and let him out. Just like with the bells, sometimes they figure out barking gets them outside and you will get false alarms.
Hope it helps.