Have you considered separating him from the others, or partition it off so it's with the others yet won't get "shoved/trampled"? IF you do put a mirror and/or a small stuffed animal in with it, a feather duster works great too. Of all the single hatches I've raised, the mirror never worked until this last one. I make a cave, supported up front with a stuffed animal for warmth. I will admit every situation gets adjusted to fit the chick.
Poulty Cell is another good product to have on hand. You could also try softening its food, adding vitamins to it IF it has difficulty drinking. Very possible it's one of those that take longer to grow (failure to thrive) but do our best to help it along.