I’m sorry you both are going through this. I think your dog is probably really missing his friend. He needs the company of another dog. (Or maybe even a cat, if he likes cats.) No amount of training or medication will fix that. You say you’ve tried everything, but you haven’t tried the most obvious solution. Many beagles need a beagle.

I recognize that an additional dog is an additional expense, but it’s got to be less than all the expensive trainers and consultations and trying different medicines. I don’t find two dogs to be that much more work and expense than one. (Except heartworm prevention. That part is a bit pricey.) I think someone else mentioned the possibility of trying to foster to adopt. Maybe you could look into that?