Separation anxiety is the word you are looking for- he hasnt learned yet to be confident by himself, which is not surprising in this covid world. I went through some bad separation anxiety with my girl, and found the booklet "I'll be Home soon" really helpful. Basically, I had to change my life for a bit to get her out of the habit of being upset when I was gone, and also desensitize her to me leaving. You have to figure out when the dog shows the first signs of distress and gradually desensitize them to those moments, for example, I picked up my keys a bunch randomly, so my keys no longer meant I was leaving immediately. And picked up my purse, etc. For you, perhaps you work on walking to the doorway, then turning back, so he never has the chance to be upset.