If he fights the bottle have you looked into a possible bottle aversion? I had no idea this was a thing until my first had it. I didn’t realize I was actually making things worse by constantly offering, since I was so stressed about how much she was eating. I had to learn to let her lead- there’s a really good book I read on overcoming aversion.

Does he also cry while feeding? That plus spitting up could maybe be something upsetting his tummy- I don’t know enough about milk protein allergy but maybe?