My canary won't sing after molting and poor diet. When will he sing?

This may be "normal" for Ringo. Molt times are only estimations based on 'most' birds. Stress, illness or diet can play large roles in molting. You can have him examined by your veterinarian to diagnose any illness or to confirm good health. Confirm proper environmental parameters and diet. There is no way of determining when he will begin singing again.

This definitely sounds like something other than molting, especially if he's very lethargic or showing other signs of illness. I would bring him into your vet for a complete exam and even possible bloodwork to rule out issues such as parasites, infections, illnesses or even nutritional deficiencies that may be causing it. Making sure to keep him warm with easy access to a balanced diet and water may help some until you can be seen by your vet.

As long as Ringo seems active and healthy, feeding him his regular food should be fine, assuming it's a balanced diet. Most commercial bird foods are formulated for optimal nutrition, and should be fine to give Ringo as he finishes the last of his molt. You can still offer the other items as treats, if you'd like.

I've not heard of food alone aiding singing, but the right nutrition can definitely help Ringo feel healthy enough to want to sing. Spending time with him, making sure he's getting enough light, and providing fun toys and other enrichment can also help encourage him as well.

Molting can last 6-12 weeks. Some canaries will complete a molt outside this time frame though. Increase proteins and fats to aid feather regrowth. Flax, niger and hemp seeds can be offered. Protein can be in the form of scrambled eggs or small crickets and mealworms. Bathe or mist with warm water to promote preening and healthy plumage. Providing full spectrum lighting, avoiding stress, limiting daylight to 8-10 hours and keeping room temperature between 68 F and 70 F will aid in molting.

Offer a fortified seed mix supplemented with cooked beans and vegetables (carrots, peas, etc). Fresh water should be available at all times. A mineral block and/or cuttlebone should also be placed in the cage. Keep the room at a steady temp and keep the cage away from strong drafts. You can mist him with warm water a few times weekly to promote preening and healthy plumage. Monitor for lethargy, decreased appetite and loose droppings. If necessary, have him physically examined by your veterinarian or an experienced avian veterinarian.

That can be a general sign of illness, especially if he is sleeping on the floor of the cage (it's hard to tell from your photo). Keeping him warm and eating and drinking can help, however since the behavior has been ongoing for a week it is best to bring him to a vet for a complete exam. Birds are great at hiding illness, so ruling out any issues such as nutrition problems, infections, illnesses, etc with your vet is best.