Does he ever have an accident when you are in the house? If so I would keep an eye on him at all times. Keep him in the same room with you, on leash or put him in his crate. Does he have a positive association with his crate?
If it is purely separation anxiety, I would work on stays and out of sight stays (very brief at first and then very gradually increasing duration). I would also work on systematically leaving the room and gradually increasing the amount of time. So first you might even open the door half way and partially hide behind the door and toss treats inside the room. Then close the door most of the way with only enough room for your hand to toss treats in. Then close the door for a second and then open it and toss a treat in. If you can maybe even toss the treat under the door. Then very gradually increase the time you are outside of the room.
Make sure he has had a lot of exercise and mental stimulation right before you leave. If he is tired he is more likely to lay down and rest and less likely to be anxious that you left him.
Have special treats/ toys that he gets only when you leave like a kong with peanut butter in it.
Don't make a big deal about getting back home. If he is in the crate maybe wait 5 minutes before letting him out. Unless he really needs to go outside then I would just very calmly without any fuss put his leash on and take him outside.
Maybe make meal time alone time. Put his crate in the kitchen/ living room. Have him go inside his crate and give him his meal and close the door. Then ignore him and go about your business. At first maybe just grab a book and sit on the couch and read. Overtime start walking in and out of the room. Then leave the room and come back in a few minuted...etc.
I hope you find these tips helpful. Good luck!
