Well done for taking in the little one. In my opinion, if he wants more and he is not bringing food back up, then let him have more. He's hardly of an age where he is going to turn into a fat kitten - "fat kittens" don't really exist. He is probably making up for lost time and food. If he is happy to take more, let him have it.
Did they give you any advice re: worming the little one, this could be another reason for him being very hungry?