I understand and can sympathies with the concept of survival of the fittest, but at the same point, i know there are times to help things along, and babies are one of those things i will help along. I had to force feed a turkey for a bit and that was a great learning experience, i think i can force feed anything now lol.
Like mentioned already, you have to physically put the food in there though. you can use any number of foods, but even boiled egg yolks mashed with honey, and maybe a bit of starter would be a good paste. prepare food, and have slightly above room temp if you can, but definitely not chilled. pick a tool of some sort, something like a Popsicle stick works well, or the blunt end of a toothpick even better. take a manageable sized glob or your prepared food paste, pry beak open with nail of one hand, you can hold it open by letting it bit the tip of your finger. that should give you plenty of room to tuck the glob of food in at the back of its beak. quickly close beak. watch for shaking. some birds will swallow without a problem, others will open their mouth and shake their head to spit the food out. if that happens, hold beak closed.you should see the food slip eventually. if you can get a couple of good meals in to it, hopefully it will come around! fingers crossed for you! give it a go, hopefully it will make it through for you..