Start by leaving him in other rooms, start slowly by going out of the room for just a few seconds and build up until you can leave him for an hour or more. This is critical; do not immediately make a fuss of him when you get back, just ignore him like it’s normal.

If you can do leaving the room, set up a pet cam or baby monitor (baby monitors are always going cheap online), go outside out of view , maybe drive around the block and wait, eventually he will stop barking, and if possible get back and come in before he starts up again. Again do not make a fuss. Get a drink put your stuff away and after a few minutes you can sit down, relax, and fuss.

It can take months to train a really anxious dog to self soothe, good luck.

Exposure therapy.

Keep doing it, and telling the dog it's ok when you get back and give them a treat. But don't make a big deal about it. Just a quick good boy and a treat while acting calm.

Have one person take him on a "walk" for like two minutes and do the same. Reward the dog and show him that brief separation isn't a big deal, then work up the time apart.

It helps if you make two "goodbye phrases". One used for brief separation, one for long. A big thing is they have no clue when you'll be back. And if you stick to basic phrase for short/long, the dog will quickly put it together.

If he knows someone is leaving but said the "long" goodbye, hell be less likely to freak out they're not immediately back

I got mine over it by inventing 'bye bye treats'.

Now he sits patiently when he knows I'm leaving, anticipating his treat for being a good boy, and not freaking out and trying to run out after me.

My FIL has an elder dog with massive anxiety and they worked with the vet. The dog is on CBD medication lol

Work with a dog trainer that specializes in separation anxiety.

Only mild anxiety, 2 minutes of barking when we left: i threw a handful of treats into the hallway just as I closed the door for a week or two.

im kinda surprised at the walk thing, I usually use the fact the dog does not want me to go without them to get them to go out. Just open the door and if they don't come then go out and close it. wait a beat and open it and attach the leash.

Most comments already said the most important things. For my dog, it took months to get her assured that we will return. We had a lot of success with doing a routine when we are leaving. We put her on the bed, say goodbye and then we put a snack or two next to her, which she isn't allowed to get, while we are in the room. As soon as I am out of the room, she is allowed, so me leaving is somewhat of a cool thing to happen. Also don't be too dramatic when you leave the dog and also when you come back. Treat it as a normal thing.. it will take time but most dogs learn it after a while. Just be patient. Good luck

I would leave small treats my dog loved, like training treats, or I would break up a larger treat, and I would leave them all over the house in the places where she would go, where she hung out. I would do this right before I would leave the house. Then she had something positive to focus on, and this treat game only happened in my absence. This made her realize there was something positive about being in the house by herself.

Mine goes almost everywhere with me. Had to install an evaporative aircon and large battery in the car for the hotter days. Fortunately I'm self employed so I can take regular breaks to check on her.

A King Charles spaniel is a rather clingy breed.

My previous dog preferred being at home.

This comment probably doesn't help you much in your instance, and others have offered useful logical advice. It's how I solved it in my unique case.

What type of dog do you have?

Mine has extreme anxiety and she's on the full dose of prozac to help. It took almost 6 months to see improvement so don't give up if you go that route and make sure they never miss a dose.