Assuming there's nothing physically wrong w/ him, does he like to play?
Does he REALLY like human "treats"? If so, tease him. I know it sounds terrible, but you can often solicit a good bark just from teasing dogs. Show him that you have something tempting, tease him w/ it and you yourself have to act like it's some big whootodo and the idea is that he'll get frustrated eventually (time varies, I'd sit aside a good 5 to ten minutes) and bark.
When he does, really praise him enthusiastically and give him the "goodie". Try again, maybe three times in a row-depending on HIS enthusiasm too. The idea is to always end it on a good note. Don't keep going until he's exhausted or no longer thrilled w/ the exercise. Know when to quit on a good one. He'll eventually realize that its something that is fun and pleases mom and should do it naturally more often when he gets excited about something and no longer just by you teasing him. In the future, if it gets to be too much, no big deal- now that he's got the idea you can begin to control it!
Hope this helps, if not, you can e-mail me.