Could you post a video of the bird and a few pictures of his feces?
There are many etiologies when it comes to seizures in birds; pinpointing the underlying cause mainly relies on diagnostic testing performed at your vet office. If a vet is not an option, the only treatment that will be able to be implemented will be very basic, and should mostly include supportive care.
The uncoordinated, spontaneous movements you're seeing suggests there is likely interference with his nervous system. Taking into consideration his age, there are several conditions that may cause similar symptoms to what you're seeing. To mention a few; nutritional disorders, either from vitamin E, vitamin B1, or Calcium can cause such problems. In addition, ingestion of heavy metals, aflatoxins, bacterial/viral infections, parasites, or trauma to the cephalic region.
Please look into some of these conditions, and consider his past history, and what condition may be more likely to occur. For example, if the duck was bullied/or hit his head that would support neurological damage. If you once saw the duck chewing a screw, that could support heavy metal poisoning.
In conjunction with a past history, performing a thorough neurological exam to test the ability of his nerve functions may also prove helpful by narrowing down what region may specifically be affected, since different etiologies can affect different areas of the nervous system. You can find a list of neurological tests here:
http://avianmedicine.net/wp-content/uploads/2013/08/17_nervous_system.pdf
Supportive care for a bird-like this should include ensuring their housing area is quiet, stress-free ( a stressed bird is more likely to have seizures). A feed and water dish is needed, but ensure the water dish is in a shallow dish when you're not around to discourage aspiration, or possibly drowning. Vitamins, E, and B play an important role in the nervous system, so the administration of those two particular vitamins, in addition to a poultry vitamin, containing a broad amount of vitamins would also be suggested. Soupy feed, scrambled eggs, or tuna are some great treats if the bird doesn't seem to be liking its normal feed. If weight loss is suspected to be occurring, daily weighing on a gram scale should be done to determine the severity of the weight loss. A 5-10% decrease in weight would grant supplemental tube feeding.