What I did with Princess that really helped was when she'd be barking the second she would stop I would say "No Barking" (in my "training" voice) followed immediately with "Good Girl", lot's of praise & even treats.
For a while it will seem like you're doing it for nothing, but he'll learn that being quiet is how you want him to be. I think you have an advantage since he wasn't a barker before, maybe now he just has jolt his memory back to the good ole days.
Good Luck!