I've heard of some folks using bandaids or gently tied strings between the legs to keep them straight. This won't need to be on long.
If he's not eating, you can syringe feed them. Some water with a bit of electrolytes may help him out. They will want company so if you can, keep him together with your most gentle chick but don't leave him with the healthier ones as they may bulldoze him unintentionally.
And as the above poster recommended, if you think this chick won't do better as it grows up it may be best to cull. There are a few ways to do it, but I'd research & pick the one that suits you best (efficiently & emotionally).