Hello and welcome!
Gorgeous dog you have there, well done for rescuing. X
As for the barking I would just mirror what others have said, big walk before you leave, treats and toys he doesn't get any time other than when you are out help too as they have the 'novelty' factor, we used to do this with our last boy and it worked for him, we would give himehis special toys and smoked bone when we left then put them away as soon as we came home so he was really interested next time they came out again.
Have you thought about rescuing a companion!?!??