This is a good site with methods of treating Dasher's problem.

http://www.ygrr.org/doginfo/behavior-anxiety.html

Might also consult with his Vet about anxiety medication for him while
he is learning that being alone is not the end of his world.It should help.
Best of luck helping him & helping yourself.

Both you and Dasher have been though a lot of turmoil in your lives and
deserve to live a less stressful life.