Even if you are overweight (which I’ve no idea if you are or not?!) and he is aiming to address that, he should be approaching it in a kind and constructive way.

When you disagree with him about other things, how is his communication style? Is he dismissive/rude/unhelpful? If so, this is a bigger issue on his part and unless he is willing to acknowledge and resolve it then you will struggle to get anywhere with fixing this.

He already massively disrespected you by messaging other girls, and now he continues to display disrespectful behaviours by prioritising flirting with other random women over your needs and feelings?

Girl, put that man in the bin. He ain’t gonna change.