Help in getting boyfriend be less influenced by his mom?

My boyfriend's mom is a first class narcissist, she never wastes any occasion to manipulate him.

I see her for the kind of person she is; my boyfriend has made a lot of progress in distancing himself from her but she still manages to get under his skin at times. Also she once texted me saying I am too good for her son and bullshit like that, always talking bad about him.

I don't want him to cut her off completely, but every time he talks with her he always looks so defeated.

Help in getting boyfriend be less influenced by his mom?
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