He needs his pack.At this point his pack is you.Dogs are very social critters.It may be time to add another dog to the mix.That way when you leave he is not alone.Another pack member/play mate may just solve your problem.
This. You don't want him to associate the crate with you leaving, you want him to associate the crate with being his own space. Leave it out and always available. Initially you might have better luck putting him in the crate and keeping the crate in your work area. This way you get some work done and the dog sees that being in the crate doesn't necessarily mean that you're leaving. Eventually you should be able to move it to a more permanent location where he may or may not see you.You have gotten some great advice here. Crate! Crate him even if he hates it. He will adjust. Do it randomly...sometimes leave...sometimes don't. Find a treat he absolutely can't resist and throw it in the back of the crate when you tell him to kennel....he will want to go in after it and he will be instantly rewarded. If you have a wire crate cover it so it is like a den and he is not just sitting there looking at you. Keep it in a separate room from where you work. I have a former rescue dog with separation anxiety...he had to make some adjustments but now he loves his crate and he will go in and out all day to take naps.
You need to be the boss...you are the dominant in this relationship. Don't be emotional. Expect good behavior and expect him to respect you. This is more of a reward for him than you would think because he WANTS you to be his leader.
He has a puppy brain in an adult body right now. If you can mold some good habits, in a year or two when he matures you will be greatly rewarded. Finding another home won't give him what he wants. There is hardly any situation where he could be with his owner 24/7 and in fact you are probably able to give him more than most in that regard. Every owner takes vacations, weekend trips, and most go to work every day.