overall, a trip to the avian vet is best needed to determine fully what the root issue is. like
@Shezbug said, if you feel comfortable sharing your general location, some of the members on the forum can help you locate an appropriate avian vet. for diet, look up how to convert him off of seeds/whatever you are feeding him at the moment. Harrison's pellets or roudybush is a great start, my green cheek, same age as yours, loves roudybush! just be sure to get the appropriate pellet size for your conure. introduce him to more veggies, having apples, grapes, and bananas all the time is a lot of sugar that can be hard on his health. use grapes, bananas, and apples as occasional treats! fruit shouldn't be given in high amounts. as a tip, give fruit when he's the most active so he can burn off the sugar and calories. for veggies, try collard greens and kale properly chopped up, carrots, bell peppers, broccoli, snap peas, green beans, cauliflower, and you can occasionally give him cucumber. does he have a proper source of light? is he getting enough sunlight? Either way, buying a proper avian uv light would probably help with his feather quality along with the diet I recommend. for fruits, try blueberries, blackberries, raspberries, but in moderation. lower intake of nuts and seeds right now, he needs to focus more on healthier things.