Try to understand specific reasons why he is reluctant to fix it. If he need more details on it provide it. If he need additional proof why its identified as issue, provide it. After all efforts if you can't come to common ground , alleviate to project status meeting.
I will be disappointed and i'll kick myself because I've applied I want to work here. i will have to follow my plan B and do better at other interviews, get feedback and not make the same mistakes again.