Was there ever any input on his quack? - which seems like a significant tell-tale sign for most of mine - though I do have a Runner, who is definitely a drake, who quacks like a girl. He always worries when everyone forages too far from the pen and starts quacking hysterically trying to get everyone to come back. And they mostly just ignore him, so he eventually joins them, but he can't do much foraging on his own because he's sooo busy looking up at the sky - commendable in many ways, but it also makes him a bit non-functional, he worries so much he doesn't eat enough and I'm not convinced he's very successful at breeding either. I think he's sweet though.
Funny how there are some the voice is hard to tell on. If my drake didn't have spectacular male saxony coloring and a very curly little tail I'd be convinced he was a she.
I'll say girl. My girls have dark bills, some mottled with a little olive green like this duck, but my drakes bills are nearly completely uninterrupted olive green (almost yellowish-olive).
eta: if it is a girl you'll soon have eggs as proof!
I think the voice will be the best way to tell for this particular duck.