check tack, try it bareback if you can, etc.
depending on where he's at in his riding, this might work, as soon as he starts backing while your mounting, get on at the same speed you would but as soon as your up there ask him to go forward. his desire to go backward and the pattern he'll establish of "as soon as they're on we go forward" will balance eachother out.
don't do it beyond fixing the problem though, because that'll make the horse go forward when you try to mount him. but in such a case the opposite can be applied in that as soon as you get on you backup.
also try to keep mounting as smooth as possible, and do lots of standing beside your horse on the side you like to mount from.
hope this helps.