Myself and my boyfriend are both slim people, but he's medically speaking underweight. I've been trying to get him to eat more food in order to gain weight, as he tends to get sick easily and runs out of energy a lot.
He has gone and got bloods done to test if there was an underlying condition, but the doctor also confirmed he's just not eating enough.

Since living together I've done my best to cook food that is healthy and tasty, bought breakfast cereal, given him larger portions, tried to make more meals a day, basically tried to find a way that worked for him to eat more.

He is a very slow eater, and so ends up feeling full well before dinner is over, and frequently doesnt finish. I dont like throwing food away so put it in the fridge or just finish it myself.

He also tends to eat lots of low nutrition food. Like eating toast, and drinking tea and coffee all day so he doesn't feel like he's hungry. 

After over a month of this I have ended up gaining weight, which isnt the end of the world, but he has actually lost weight.

I got frustrated with him and now I want to just prepare my own meals from now on and let him do what he wants, because this method isnt working for either of us. 

He got upset at this and said he was trying, and that I wasn't being fair, but I dont think its my responsibility to be monitoring his food intake and constantly trying to find meals that I think he'd want to eat, while putting my own desire to eat a differing diet in the background 

AITA for deciding that its on him to care for himself and not on my shoulders and wanting to just do seoerate meal prep? 
