My 9 month male has started trying to mount his mother, who has recently been speyed.  He hasn't quite got the knack of it, so nothing has happened so far.  The mother was dressed to prevent this problem (although no pups will result now) so is this normal and will the mum be hurt at all if he keeps trying and eventually manages it?

This is normal, and due to your dog having extremely high testosterone levels at the moment as he is an adolescent. This will settle down as he matures. Also many bitches are attractive to dogs for a short time after spaying because of the physical upset to their system - hormones can be released when the ovaries are handled and removed. This also will wear off. But yes, both your bitch and dog could be seriously hurt if he succeeds, so prevent him from attempting.