My dog won't sleep laying down

Ok, odd title. A few days ago I posted a thread looking for an emergency groomer for my husky/mutt. He had some matting, not excessive, but it was deep. So we got him groomed and it's all good.

Before his groom, he would lay down and immediately sit up. He's never done this before. We thought it was just discomfort, hence the emergency groom, but that doesn't seem to have solved the issue. He sleeps at the foot of my bed, and continues to sit upright as he dozes off.

Couple of things to note - he doesn't whimper or whine. He has no facial expressions (other than exhaustion). No heavy breathing. He doesn't try to go for any specific spot on his body to indicate discomfort.

So now I'm thinking it's a medical issue? He's otherwise *very* healthy. But maybe someone here has experienced this before? He seems like an insomniac, but it's come very suddenly.

Edit: we took him to a derma vet last month. He had his bloodwork done, nothing out of the ordinary, thyroid all good, no mites. His stool is also very healthy.

Any advice is very appreciated, I want this eternally happy soul to be comfortable.