Listen, I'd say either have him have a good reason to act this way on his backstory, or develop his backstory totally unrelated to this mannerisms. I said to go for the extremes bc what happens is that most of our mannerisms were either aquired due to a certain experience that had an impact on us (positive or negative, up to you!), but our mannerisms can also have been aquired just bc we see someone we admire/our social group doing it, so we end up doing it ourselves. I'm not gay, nor a man for that matter, but my main tip would be, have the backstory relate to it, but not the plot, avoid making all the jokes about his mannerisms, all his reacions tied to his mannerisms and add a pinch of paradox (for example, have him like mostly affeminate and flamboyant things, then a random manly man cishet dudebroski kinda guy is listening to heavy metal and he joins him, having these few tonal breaks help avoid making a character too stereotypical). Another example would be a super dark, manly killer-for-hire widowed doberman dude I created, who actually likes to have cook and be in sunny places and picnics and had his wife propose to him. Still, this might be personal preference, so take my advice with a pinch of salt. I hope this helps somehow! :)