Without knowing what the fetish, it's difficult to be able to say too much, but completely respect that you might not want to disclose it.
That said, it's entirely up to you whether you want to engage in a specific sexual practice, whether as a one off, occasional treat for your partner or every day for ever, or simply never.
I am in the camp of "try anything at least once" as I think you never truly know if you like something unless you have tried it, but everyone will probably have something they will never try.
Only you and he can decide if you not doing it either occasionally or never is a complete deal breaker. It may be that you are happy for him to indulge on his own, or with someone else, but that may carry its own risks as well.
There are plenty of people on here, and in the wider world, for whom missionary once a month as as much as they want/need and some people want/need to be fucked senseless, use toys, whips, tying up, being in a 20 person orgy 3 times a day.
No one is wrong, it just needs a discussion and agreement from each of you where your own lines are drawn!