When you are going to get on, take your right rein and take some of the slack out of it, so he can't reach his nose around at you.
You could put a cavesson on him temporarily too, so he can't actually get you...sometimes they will stop if they can't physically manage to do it. But, that is just a temporary fix, and won't stop the behavior in and of itself, usually.
I will have to try and dig out some of my books, as I've never had to deal with a horse that kept up the behavior beyond one or two half hearted attempts. Usually a knee, elbow, or crop to the muzzle as they are attempting stops it cold. Use the crop as a 'barrier'...and he won't be able to evade it like your hand...he goes to bite, he meets the crop. If he doesn't know to keep his head out of your space at other times, you may need to teach him that, as essentially when he is doing it when you mount, it means he's not regarding your space in a respectful manner.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."