Duck bright green poop

[Cocpn]

Welcome to BYC.
Bright green poop can be indicative of an anorexic state; considering you stated he has not been eating much, and the fecal matter portion of the droppings are very scant in the pictures. I would assume that would be the cause of the green coloration.
These are all non-specific signs/symptoms we can't narrow down to a few conditions. All I would suggest of now is to bring him inside, or into a warm, stress-free area outside where he can readily access feed and water, and you can monitor him better. I would first weigh him on a gram scale, and continue to do so to watch out for any weight loss.
Generally, when there is a 5-10% decrease in weight, one would want to implement supplemental tube feeding.
https://www.backyardchickens.com/threads/tube-feeding-ducks-updated-5-7-2020.1211994/
If he is drinking, you can try offering an electrolyte such as Gatorade to help ensure he stays hydrated, and to correct any electrolyte deficits.
Your best route figuring the underlying cause would be seen through a vet; if you can't, you can try posting a thread on the duck forum, and try to provide some information that could be helpful (physical findings, past history, housing/feed setup, etc)
https://www.metzerfarms.com/Veterinarians.cfm
https://www.backyardchickens.com/forums/ducks.42/post-thread

Thank you so much for your help really appreciate it.