I have the same problem with ds6. Not an ounce of fat on him but eats like a train, breakfast, morning tea, lunch, afternoon tea, dinner, dessert and snacks all day in between as well. I've checked for worms lol, but he has obviously a high metabolism like me. I'm at a loss sometimes to find extra's apart from junk and more fruit.
I like the pancake (veggie patties) idea
I'll be watching this post for more ideas.