This whole thing seems so damn weird. Do you live in a particularly conservative area where open bigotry is accepted and encouraged? Because in most schools these days someone being gay is accepted pretty easy and the student body would be far more likely to tear him a new one instead of you. Also his mum is bi, what the hell?

But okay even if it was an ultra conservative area where gay people are met with massive ridicule, wouldn't the homophobic friends look at him sideways for willingly doing the dare for several months and making out with you?? It definitely sounds like his hostility and mockery of you might be performative to convince his friends he doesn't swing that way.