Nowt beyond what you have. Be nice to whatever bf/gf he bring home, it won't last, it never does at that age, and since he's old enough to be thinking about it, read him the riot act about proper sti/pregnancy protection for when he gets around to doing it.
Basically exactly what you'd do for a monosexual teenager, but with a bit more flexibility in the pronouns.
Quick point that stood out for me: KissesBreakingWave says "it won't last, it never does at that age".
Don't tell him that.
I'm bi, told my mum at 15 and she said it was a phase. Totally heartbreaking to have anyone tell me how I should be feeling or that what I was feeling wasn't real and to hear that from my mother? I stopped talking to her about it and as luck would have it I have a boyfriend now. I've avoided being in gay relationships because I knew I didn't have parental support and simply couldn't picture how I'd bring someone home.