I'm in my late twenties and my brother is in his late teens. He has always been a skinny kid so recently we decided that I would be taking over his physical development. He explicitly told me that he wanted to get bigger and would be willing to follow my guidance.
I set up a system where I'd be tracking calories and preparing meals for us both. It's a simple thing but very time consuming, so my plan was to introduce him to these tasks gradually, eventually splitting the load with him 50/50.
We started out well enough but within a month it devolved into me having to interrogate him every day on his weight and whether he'd eaten all the food I prepared. Most of the time he was unenthusiastic and had some excuse as to why he hadn't.
At the peak of my frustration with his inability to appreciate my efforts, I gave him a heated monologue about how lazy and ungrateful he is and that basically if I'm playing the housewife the least he could do is eat the food I'm preparing. He took it well and improved for a couple of days but quickly went back to his old ways. I have since stopped preparing his meals or reminding him of his other chores and our communication has become tense.
He's my brother so I don't want to have a massive falling out over this thing, but continuing in this thankless duty is probably more than I can handle in the long term. An outside perspective might just be the key to resolving this optimally.
I am worried that I am coming on too strong in this. How do I confront him about this intelligently and really ascertain his willingness to keep doing this?