This is a male (yellow-shafted) Northern Flicker. There is a green tab at the top of the page that has advice on how to deal with injured birds. The fact that it was unable to fly means it either has a brain and very likely wing injury. It could also be that it's lost its flight feathers and can't fly. Either way, leaving it outside would be leaving it for dead I think. Their main food source in the wild (from what I've observed) is actually ants and beetles. I'm not sure if it will eat the seeds you've provided, but maybe. It's obviously incredibly stressed, and you should keep it in a dark space with ventilation, and contact your local animal rescue shelter ASAP. Either he'll just need time to regrow feathers, or it's (likely) much more serious, and there might not be much hope for this poor little guy. Keep your cats indoors, a study in 2013 found that outdoor cats kill between 1.4 and 3.7 BILLION individual birds each year in the US alone! Even if it's not your cat, maybe talk to your neighbours.