I think I am going to make my homophobic friend's family think he is gay
That sounds like a bad idea. Does he know you're going to do that?
Yes, I have discussed it with him
It's understandable to have fun playing pranks, but its wrong to pretend to be something you're not. It's also insulting to minorities that misrepresent them.
Who cares about being insulting? I'm going to do it!
I'm sure the minorities you're misrepresenting care about being insulted. Making them the butt of your joke shows that you don't respect them or the struggles they've had to endure. Why not come up with a prank that isn't at other people's expense?