I use something called peepers on my chickens when they start to pick on each other. A peeper is a little piece of plastic that covers there vision so they cannot see in front of them but can see to the sides and down this way chickens can't take runs at each other as easily.
I just grab each chicken individually and tap the nipples with there beak then I leave the rest to them. I have never had any problems with dehydration.
Try force feeding with an eye dropper, if that doesn't work then you will have to weight and see. Make sure that your brooder is warm enough 85 F to 90 F Is the temperature he needs to be kept at all times until he Is old enough.