I recently wrote this out to a friend in a similar situation:
First you need to locate the nest. If it's out of reach you can at least observe what the bird is which is critical for figuring out the diet (watch for the parents). Find a cup-like thing, or a bowl, pad it with a washcloth/something soft and line with paper towel as you'll be changing this a few times a day (!). They need to be propped up a bit for proper development of the legs or...something. What's the development like? More bald? At this stage they can't thermoregulate themselves very well so he may need to hang out under a lamp. That said it's easy to overheat them so keep an eye on him if you don't have a thermometer handy (panting would mean move him asap).
Diet: At this stage he's going to want food every 20 minutes or so until about 9 at night. Soaked dog/cat kibble (soaking in Gatorade this time is wise to get those electrolytes up), hard boiled egg yolk, crushed calcium, and avian vitamin supplements. 27% protein and 13% fat is the ratio you're going to aim towards when searching for some dog or cat kibble to soak in room temperature water. The consistency should be like canned food but I personally like it a little more moist just because it can get stuck in their mouths. No water! This will kill him. The water soaked up by the kibble does the trick. There are two pathways in the bird's throat and the one underneath the tongue is a one-way route to his lungs so pure water will drown him. His tongue will direct everything else the right way. I find it easiest to just get a little bit on the tip of my pinky finger and give it to him that way other than a feeding stick to reach the crop. Mine's small but my starling would swallow my finger whole...kind of gross at first. When you get the time, mash up hard boiled egg yolk, some applesauce, and crush some calcium Tums into a fine powder. This is the diet I raised both starling and sparrow on but this swallow will require insects (categorized as a meat diet rather than grain like the other two). Another thing to note is anything remaining in the throat near the airway can cause a bacterial infection so if there's anything left over after several minutes you may want a Q-Tip to wipe it out. Oh! And wash your hands before and after.
At this age they're fairly trusting so you should be okay getting him to open his mouth for you but if not have your hand approach him from behind his head and chirp at him. You're going to feel a little silly but you're just mimicking the mother. Kissing sounds and tapping the beak may work too. If he's older and thus more afraid of you you may have to place a bit of Gatorade on the sides of his beak and let them leak in. This should trigger (eventually) him to open his beak even for a moment and you can catch him there and gently get some food down there. He'll quickly learn to trust you when he associates food/survival with you.
I'll fire this out now since you posted this 6 hours ago (sorry if it's disjointed). Good luck! It's kind of scary at first since they're so helpless but you can do it.
Edit with less important details: It's best to find an experienced rehabilitator in the area. And I stress the rehabber part because most wildlife shelters don't have the manpower or interest to put so much effort into saving "insignificant" species like these and will euthanize them. So thank you for doing this.
Unfortunately most are full at this time of year so it's best to load up on info assuming there's a wait. I had the starling for two weeks before finding someone who could "wild him out" for me. With last year's sparrow I never found anyone (though I was referred to a person four hours away!) so I raised and released on my own which is an entirely different issue. For your own protection don't offer your name or any personal information as in the US the laws are pretty strict about interfering with migratory birds (that said, Canada's pretty hardass about it too). If you get to the stage where it looks like you're going to be the one to release I can guide you from there. But for now warmth and blood sugar are the key things.
His feces are a good indication of how hydrated he is. Ideally you should get a sac - if it bursts open (sorry guys!) he's pretty hydrated! The skin is another way to check. Baby birds are ugly and wrinkly but too wrinkly means he's in trouble and needs fluids ASAP.