It depends, is it just reactivity to outside stimulus?
Or, is he standing in front of you or to the side of you looking at you and barking?
Or, is it separation related? If you leave how long does it take for the barking to start and how long does it last?
The reactivity could be helped by more exposure to different things in a postive way.
The pushy "look at me barking" I've either just turned my back or put a book in front of my face. It's not instantaneous but eventually the dog gets it.
Any attention is still attention whether it's negative or postive. With Alice, even tilting your chin slightly in her direction she would continue to bark at me.
A less intelligent dog just turning your back works; with a GSD or other herding type dog...they just circle around so hence the book or magazine.
Separation issues are tougher, start with the time issues. When you leave, time how long the window of separation is before the barking starts. Work from there in small increments...
When arriving home or leaving don't make a big deal about it. I've found that ignoring her for a few minutes upon arriving and just leaving without a word has really helped.
And that's what worked for me with Alice.
Zoey, the weasel is old and barky reactive and I'm personally thinking of a remote control citronella collar. As she sets off the younger bigger dog.
It's really tough with her because she's elevenish and was given high value food rewards and attention for reactive and "dominance/demand" barking. *thanks mom*
Edited to add: Exercise! Exercise! Exercise! Mental and physical! A tired youngster doesn't "yell" about everything so much.