He's someone who's going to be a part of you for a very long time that if he continues to keep on mocking your stutter like this then it will have severe long-term effects on your confidence and self-consciousness which in turn will be damaging and diminishing. A true parent figure would just ignore it and understand that you're trying your best and not make such a huge fuss about it unless you'd like any tips on reducing your stutter through therapy but what you do with your stutter is entirely up to you. Do communicate to him about it so he sees it from your side of things that you can't be able to flourish and be happy if he's going to unintentionally come across as such an asshole.