Yeah, we've already done the shopping for a good while of an existing meal plan

Not wasting that money, so just adapting where I can and giving him something else when I can't at the moment. I think its probably easier with an adult than a child because as he says, he can just eat mostly meat and certain salads/veges and be fine without pasta, bread, etc. He claims he's happy with next to no carbs but he's always been a carb guy, so we'll see how that goes. He doesn't seem to understand the diet at the molecular level though, so I can't let him do the shopping or choose himself anything to eat. He'd have bought himself commercial rice bubbles which have gluten because he won't read labels.
I'm in two minds as to whether to cut the things that are on the suitable list but "may trouble some people"
I have downloaded a few things off the Monash site and have trawled the internet for recipes to adapt my existing repertoire. The thing is that there won't be much variety because he objects to most of the alternates (won't touch soup, porridge-like cereal dishes, etc.) though I guess that matters little. He's not a foodie by any stretch and he seems fine with the idea of this diet.
I wonder if this will help us work out why he's so skinny and help him put on some weight.