Sounds like he may need special care He will need plenty of heat 36°C if he is still very young (less than a few weeks old). The droppings look like he has been starving or has salmonella bacteria infection. I would take him in, give him heat and handfeed him. I also would have a pn avian veterinarian diagnose him as there are several causes that can be a factor in addition to starvation or undernourishment.
One can buy a quick salmonella test kit from Jedds.com you can test for it yourself in about 10 minutes. The test can be bought at
https://www.jedds.com/shop/salmonella-test-kit/ but his droppings are somewhat intact, so it may simply be from starvation, but it will be good to rule out if he does not have an infection.
The green color can also be due from liver dysfunction, so only give antibiotics if positive for pathogenic bacterial infectiom, as antibiotics may overload the kidneys and liver and cause death especially if suffering from malnutrition, stress, or systemic internal illness, and not every antibiotic will work and could only do more harm if misused. Also, the bird will need probiotics after antibiotics.
The watery urine is called polyuria. This could be from starvation, kidney and or liver failure, or even from viral infections like circovirus and herpesvirus. Supportive care will be needed immediately. Heat therapy, electrolytes (Pedialyte for infants will work and used only to replace water in formula not given directly down crop), vitamin A (single dose of 250mg/kg added to formula if formula does not have vitamins), and calcium-enriched baby bird handfeeding formula (with a few added drops of Omega-3 fish oil and a drop of cod liver oil to approximately 250 mL of formula).
Keep the bird calm in a dark warm environment with a nest bowl with padding so the legs do not splay outwards and remain under the bird. Use a heating pad but make sure the temperature is no more than 36°C. If the bird is 3-4 weeks old and has juvenile feathers, you can keep it between 27-32°C. If the bird starts panting you can lower the temperature slightly, but still keep warmer than room temperature.
Feed every few hours only when crop is empty from prior feeding.
I hope he gets better and this information helps.
