Hi and welcome!
As you‘ve said, separation anxiety is a common problem and there are lots of tips for helping to deal with it and its often a matter of trial and error to find the ones that work best for your individual dog.
My advice would be to go back to the very beginning and get him used to you just popping in and out casually for a few minutes, don‘t say anything like „stay“ or „won‘t be long“
, don‘t even give him eye contact, go out and come back in after a minute of so. If he‘s quiet you can give him a treat but try not to make a big fuss, remember you‘re trying to make your absences a „normal occurance“ for him, it shouldn‘t seem like anything put of the ordinary.. If he starts to howl, try and wait for the second he‘s stops and then go back in and treat, avoid going in when he‘s making a noise, he needs to learn that making a fuss won‘t get your attention, if he howls and barks without a pause, fo back in but completely ignore him. Keep your absences very short to start with, just a minute or two and then build it up very slowly but stay as calm and matter-of-fact as you can, they‘re so astute and will pick up on any bit of anxiety you show.
It often helps to limit the access to just one room where they feel safe (liken it to being alone in a big strange rambling house, you probably feel safer in the sitting room with the door closed!
):
Try and make sure he‘s fairly relaxed when you do the training i.e. fed, walked and toileted. A piece of your clothing, a favourite toy, some background music/television are all things that you can try try make him feel more secure and possibly somewhere where he can „hide“ - my dog often lies with his head under a cushion
, or squeezed under a low table - maybe you can leave his crate somewhere so that he can go in if he wants. ?
Hope you can sort it, I know how stressful it can be and it becomes a bit of a vicious circle as you start to worry about leaving him and he knows there‘s something up.
I‘m sure others will be along soon with some more tips. Best of luck, let us know how it goes!