What breed is he and where did he come from, out of interest? Separation anxiety is utterly draining (as you know!) and would be a common reason for giving up a dog of this age, I'm just wondering if this is more than just not settling in and is a pervasive issue.
As it is I would introduce a crate and go for gradual withdrawal - set it up in your room and have him in there with you for as long as you can stand, then gradually move him out to the landing and then the spare room or kitchen, perhaps leave the crate door open but block up the doorway with a stair gate so he can still see and hear you.
Normal crate training rules apply, never force him into the crate, get him to see it as a safe place with soft beds and nice treats, feed all meals in there, settle him in there after walks and training (so both brain and body already tired) with a nice big bone/chew and pop in and out of the room at random intervals, just a few seconds at first, make sure he can still hear you moving about the house.
Same for leaving the house, desensitise him to any cues by putting your shoes on and slamming the door at random intervals without actually leaving, then only leave for seconds before you come back, gradually stretch it out to minutes, it can take months to work it up to hours. Use stuffed kongs, raw marrowbones etc. to distract and keep him busy. Stair gates are really useful, set a couple up throughout the house so he's unable to follow you from room to room, he must get used to settling on his own for short periods, although initially being able to see and hear you in other rooms through the gate before you start pushing the distance and time.
I don't think any of the calming stuff works - I know both DAP and Zylkene have been proven to have an effect in trials but I personally didn't see any difference with a truly anxious dog, true separation anxiety will not respond to pills alone and needs systematic careful training to overcome. I always
when people trill "have you tried leaving the radio on for them while you're gone", it's a little more complex than that.