would you recall if the tiny one had a difficult hatching? There is always a BIG one and a SMALL one in any hatch - the rest seem to be sized in between. Is the little one getting enough to eat & drink, or do the big ones shut him out. I have a feeling he isn't getting enough and the struggle with the others makes him too tired to even try.
He would probably do better away from them, they probably run into him or trample him several times a day which really wears him out. Get him some Polyvisol infant drops (NO IRON) and give him 2 drops 3 times a day - on food you know he will eat, so put the drops just inside his beak so he can decide to swallow them if he doesn't like it.
He would probably like hard boiled, finely mashed egg yolk moistened with a bit of plain yogurt.
If you separate him to a quiet, warm area he is likely to gain some strength & energy. If he has no injury to legs or feet, he should gradually develop control over them. Give him a small stuffed animal to cuddle with, and if he is lonely a little mirror so he thinks he has a friend. Make sure he has a warm end and cooler end to his enclosure, so he can lay where he feels most comfortable.
If he has a water dish put clean stones or glass pebbles(from hobby store) in the dish to prevent accidental drowning . Do you have a photo of him so we can see what's going on there? When you have the chance carry him pressed against your body- to keep him warm - the movement will stimulate his circulation. Chicks that get special attention often end up the most sociable and baring any internal problems he can eventually catch up to his siblings in size and vigor. I hope you will give him the chance.
He may need his chick starter food ground up smaller so he can eat it. It's common in bantam breeds to do this until they are big enough to handle regular pellets or mash.