Thank you for clarifying that, it makes sense now.
I suspect that he is freezing because he is in pain, having moved just the wrong way to set off a pain response and he is focused on that discomfort and is unable to concentrate on anything else.
Prednisone is a steroidal anti-inflammatory which I assume they were hoping would decrease local inflammation, especially to any nerve roots in the area, and make him more comfortable.
Gabapentin is a pain reliever but I often use it with other pain drugs as it seems to help with "wind-up" nerve pain, hopefully stopping the pain before it starts with nerve root related pain, but is not as effective at stopping pain once it starts.
Whether to take him to an emergency clinic tonight or not depends upon how uncomfortable he is, meaning how many episodes has he had, and whether you can get him to lay down and relax.
If he is examined at emergency tonight they may be able to add another pain medication, probably Tramadol, to get him more comfortable.
The other option tonight is to try alternating hot and cold packs on his shoulder to try and relieve his discomfort that way, and then have him seen by his regular veterinarian tomorrow morning.
Please let me know if you have any further questions.