I believe they have citronella collars for barkers. Have you thought about that.
I know it must be frustrating, how can you stop a problem when you are gone.
I believe the dog whisperer would suggest that you make sure he is worn out when you leave. Play some fetch or go for long brisk walk before you leave.
You might find some information about it on his website.
http://www.dogchannel.com/dog/expert...aspx?WT.srch=1