I do not know your answer, but a possibility is that he has subluxated vertebrae in the spine, which might causing enough pinching of the nerves that he would be temporarily 'paralyzed'. If so, this could explain why it comes, then is relieved.
This can happen with people many times with sometimes only minor 'bumps', and it happens to animals also. It may be painful, but does NOT have to be as a symptom.
I do not know of a chiropractic veterinarian but there are a few chiropracters who might be willing to treat your pet. Probably x-rays are necessary to determine if subluxation is present, before treatment should begin.