I would be worried about a possible neck problem.
Spinal problems can include trauma, compressed or ruptured disc, even tumors
Pressure on the portion of the spinal cord in the neck can lead to difficulty/pain in moving the head (side to side and/or up and down) a stiff or weak gait, and problems using the front limbs, including difficulty supporting body weight.
Trembling can be due to discomfort and/or anxiety.
This does not sound like a fractured rib to me, but it does sound like he needs a complete and thorough physical and neurological examination as soon as possible. It is possible your veterinarian will want to take xrays.
Any time there is possible pressure on the spinal cord it is an emergency, as even a small increase in such pressure can lead to paralysis
Treatment depends upon what exactly is causing the problem and how severe.
See the following:
Do not give any medication (especially for pain) as your veterinarian may wish to administer medication and this could interfere not only with his interpretation but with treatment. Hope this helps.