what happens if U are at home, but tether him or close a door between U, leaving him in another room?...
with a place to lie comfortably [a bathmat, folded towel, his bed] & something to CHEW,
such as cow hoof, antler, solid-white Nylabone, or other busywork?
does he relax & get to gnawing, or whine, bark, pull at the tether / dig at the door?
There's no real way to distinguish between genuine, full-blown Sep-Anx & sheer boredom, except that Sep-Anx is driven by a desire to ESCAPE, sodamage is mostly to door thresholds, floors at exit doors, window-sills, & other possible exits,
while destruction to fill the time, self-soothe anxiety, or out of boredom, is directed at anything chewable.
Does he fuss if U are in the shower, with the bathroom door closed? - many overattached dogs want to lie on the shower mat, outside, lest U somehow escape the room while they're outside it.
I would not use or recommend a wire-crate; i'd buy a used airline-approved shipping crate
, & HABITUATE him to it over a week or more; he'd eat all his meals in the crate, 2x a day, every day.
There'd be a durable chewy in there, attached to the window with a length of bike-cable so he couldn't take carry outside the crate - a thick-walled pork marrowbone, sterilized
, with the cable run thru the hollow center, would be good.
I'd put his meals into black Kongs, one size larger than recommended for his body-size [a Lg for a medium dog, an X-Lg for a large dog, etc], & secure the Kong with a length of knotted nylon cable, a BIG FAT KNOT inside the Kong under his food, & a SNAP on the end of the cable, tethering it to the crate window so he consumes it inside the crate.
To wash the Kong, unclip the end, carry it rope & all to the sink, use a spatula to dig out the remaining food, & slide the knot out; put the whole thing, rope, Kong, & all, in the top rack of the dishwasher, making *sure* the rope & the clip cannot fall into the arms of the sprayer!!
A reusable cable-tie or a couple of rubber-bands - not just one, in case it pops - will keep the rope coiled loosely to be washed, & safe from tangling.
If U don't have a dishwasher, soak the Kong & rope, submerged, in a large bowl or water, to soften all the clinging bits; then spray the whole thing well, to blast off stuck-on food, & finally, use a bottle-brush on all parts, including the twists of the rope; let it air-dry B4 use again.
[If U soak it well, actually scrubbing it takes maybe 5-mins. Air-drying takes longer.]
If his problem is Sep-Anx, it may take more than advice over the 'Net to repair - he may need an SSRI to reduce his anxiety, or other Rx meds; he will definitely need a B-Mod plan from a vet-behaviorist, CAAB, or certified canine consultant
[IAABC world-wide; COAPE in the U.K.; other credentialed pros, depending upon where U live].
Vet-behaviorists are thin on the ground; CAABs are more common.
Both may offer remote services, using Ur own vet as the locum for hands-on exams, & U as the owner will do the B-Mod per plan, & report on progress - they may see him via live-feed / FaceTime or other, or see his behavior as it was recorded.
________________________ I would also suggest U get his behavior when home-alone using CC-TV & video saved to a dedicated flash-drive, or a Cloud account.
BACK IT UP - don't count on one format; Cloud can be corrupted or deleted.
A flash-drive plus a DVD, or any other hardware storage, plus the Cloud, is safest.
CC-TV cameras can be $30 or so; all U need is WiFi, to connect them to yer laptop or desktop.
Seeing his behavior in real time may give U a much better idea if his focus is escape, or filling the time / relieving anxiety / alleviating boredom. Hopefully, it's one of the latter 3.