Sounds like pretty full on separation anxiety related problems, in a dog who already has anxiety/stress related problems, who is also a teenager, with all that entails.
It won't simply get better by itself and it won't just 'go away', I think you know that but you will get people telling you to just leave him a few minutes at a time and he'll grow out of it... etc.
I would strongly advise you to seek help re separation related problems, there are a number of specialist behaviour consultants who deal with this (I am one of them, but full client list right now) and their support is vital, it is a long and tedious process and you do need someone elses support to get through it. The work itself is not difficult, but it is slow and boring.
To give you an idea of how slow - one of my current clients is doing AMAZINGLY well... in 4 MONTHS we have gone from her howling at the door should the owner step out through it briefly, to now 8 minutes of chilled 'have a look at the door, wander off, get comfy somewhere else' whilst owner leaves, sits in the car, shuts the car doors, runs the engine.
To fix an anxiety related issue of any kind you need to remove as much stress/triggers for anxiety as possible, and then implement a behaviour modification program, generally desensitizing gradually to the problem situation.
That often isn't easy and generally people go far too fast (see: 'leave the dog for a few minutes...' in many cases these dogs need to do lots of work before you even GET to any absence at all and when you do, it is a few seconds, not minutes!) which of course makes the issue work.
To address SRP you need to ensure you never leave the dog exposed to the fearful trigger, in this case, being alone, as each time you do, you'll undo the work you've previously done.
That is sometimes impossible for some owners, it generally involves a lot of organisation, sorting out sitters, family members, professionals, daycares etc - and of course is also often very expensive.
If you can't do that then rehoming may be the better option, however people are not bashing down rescues doors for dogs with SRP, and rescues are pretty full so its best to start on the process even if you're considering rehoming, as at least it won't make the issue worse.