No, from the description myositis is highly unlikely. He claims his leg was paralyzed completely, not just the calve. Also rapid onset, no phase of "weakness", equally rapid recovery of function is inconsistent with significant muscle destruction. OP didn't report pain, or changes in perception and sensitivity, but I suspect the latter might have taken place.
I'd pick something neurological. Diagnostics involve a lot of poking with needles and electric stimulation, much like you would debug a circuit.
Does the muscle work when stimulated directly? Does it do something it shouldn't while not stimulated (spasmic activity). Do the nerves leading to the affected muscle conduct direct stimulation? Are relevant reflexes working?
Today you'd probably also use an MRI do rule out lesions in the brain and spinal chord. And still, after all of that you might not find anything really "wrong", which often is a good sign.
One case I witnessed had virtually no motor function behind the hips, and no control over bladder release and completely recovered within one or two weeks without significant intervention.
Unfortunately the owner didn't want to pay for an MRI, but the dog, which had a long history of coagulation problems anyway, probably experienced some kind of bleeding somewhere in the spine. Such "vascular" causes typically happen and unhappen that quickly.
One acronym is VITAMIN-D: Vascular, Inflammation, Traumatic, Anomaly, Metabolic, Infection, Neoplastic and Degenerative.