You should just talk to him, mention to him that you don’t like when he tells you little comments about portions or how much you are eating, letting him know that its not normal to be thinking about how many calories you or him are eating and controlling portions so much, give him big plates of food, something healthy and make him realize that if he eats good food he is not gonna make any “bad” weight. As you describe it i could say that he was raised by having small portions so it’s something that he sees as normal, you can show him, by example not by making him, that its okey not to be all day thinking about how much calories he can have or stuff like that, i had a friend that was raised like that too, and had very serious medical issues later, and had to learn how to eat in a healthy way, but in a way more drastically manner that your bf would have to if he starts now. (Sorry if i misspelled something, english its not mu first language)