My [22F] boyfriend [23M] has a hitler fetish and I don't know what to do

I [22F] have a boyfriend [23M] and he won't stop asking me to dress up as hitler. Throwaway so he hopefully doesn't find this... It started innocently, I was tickling him and he called me a hitler to say I'm evil. He said that tickles are the absolute worst thing for him because it's his weak spot. As the days passed, he would make this joke more and more frequently. Eventually he started moving my bangs to the side and putting his fingers above my mouth to emulate a mustache. I thought it was just an insensitive joke but he keeps talking more and more about it. Yesterday he asked me if I would consider getting a uniform and later I peeked at his phone and for a second saw hitler cosplay in his searches. This came out of nowhere, he's generally a sweet guy and treats me well but this thing has me concerned. He tries to reassure me that he doesn't support any of it and just thinks it's funny.

I don't know what to do, please help.