Here is what I do when I feed my little Poppy.

I close him in a room and put just enough food in his bowl that I know he will eat it all in one sitting. I have learnt this through trial and error. At first when he didn't finish it all, I would put it up high somewhere so the bigger guys wouldn't get it. Then later I would bring Poppy back into the room and feed him a little bit again. He gets about 4 feedings a day like this. Morning, afternoon, around supper time and then again at night before bed.

Maybe you can try feeding little Sushi like this for the time being.