There are a number of things it could be.
Anytime a dog is sick, he could become weak in the hind end due to weakness.
An infection could cause him to be weak.
Tick borne disease, such as Lyme's disease, can cause weakness in the hind end. I am not sure if ticks are out where you live because it is winter time.
Anemia, or not having enough red blood cells, can cause weakness. There are infections, parasites, and auto immune diseases that can destroy red blood cells. Sometimes a defect in the bone marrow can cause a deficit in the red blood cell count.
He could have a spinal lesion where he has a inter vertebral disc that is bulging, or herniated out, and it is putting pressure on the surrounding structures and nerves.
He could also be having degenerative joint disease (arthritis) in his hips or spine. You can try sitting behind him and pulling his hind legs straight back towards you until they are nearly at the level of his hips. If he resists this, he may have lower back pain or hip pain.
The trembling could be from weakness or it could be from pain.
To get an official diagnosis, I think he is going to have to be examined by a vet.
I hope this helps.