Question about orphan runt of the litter kittens?

Hi i'm unofficially taking care of 3 orphan kittens ,the shelters did not have any place for t use hem and The waiting list for official foster families is long . Well they are supposed to have 8 weeks but i think they cannot have more than 6 or 7. they eat and use the cat litter er tray,but one of them is really small and he eats less than the rest .he plays around and fight plays is sister's but i'm worried . His he out of the woods as he eats by himself of should I force him to eat more? I notice he doesn't have the same interest in food ,i have to put him in front of the bowl 2 or 3 times until he eats .