You could try giving him a long lasting treat before you leave, I like frozen kongs or licky matts. It’s enriching and will give them a positive association with being alone.

That is so frustrating. You know your dog isn’t being bad but you don’t know how to fix it. I just did an article about this. Give this a read good luck.  https://www.darwindogs.org/post/dog-training-separation-anxiety-cleveland-puppy-behavior-positive-easy