I have 14 year old also fussy and skinny. I also feed him once a week, and use it as an opportunity to give him more traditional meal, within his preferences- I want him to enjoy it. But I sneak a little something new now and again. This week he ate the same amount of dinner as I did, gammon joint slices, his favourite mash and baked beans. Then a pancake, and a little later, a Magnum. He also asked if I had a crime egg. I failed on that one. He also drank a lot if Schloer. If there is no weight problem I will let him have anything I've got.