The key is to make sure you do not reinforce the barking by taking him out (giving him what he wants) when he barks. While you want him to let you know when he needs to go out, you don’t want him to think that if he barks he gets to go out. When you take him outside, and he does his toilet business, you should positively reinforce the “potty” experience with excitement and “Good Boy!” and high praise for successfully pottying outside.
I’ve got that part down. But, like dealing with a crying baby, the first thing you have to do is eliminate the obvious source of the problem.