Assuming he is not too cold or conversely too warm it is likely suffering from overgrowth of flagelettes in the gut. These are one celled parasites commonly contracted from dirty water or alive in a food item. Common cure is Metronidolzole aka "Flagyl", a brand name. It would be best to supply a vet with a fresh, wet stool sample to look at under a microscope. If present it looks like everything is totally spastic as they move.