You can teach him firstly to stand, by giving the command and gently correcting him each time he tries to move off and replace him until he gets the message.  Then you can apply this to standing by the mounting block (but not mounting) until he is comfortable to do this at any given length of time. 

Another thing you can then do is mount or even half mount (with someone holding your horse as suggested before) and then dismount immediately, and then repeat this until it becomes no big deal.  Some people I know rein-back their horse one step immediately after mounting, but I think the main thing is to make sure the horse doesn't move off the second you are in the saddle.  Some horses are so well trained that the rider doesn't even have to hold on to the reins!

Check the saddle, and his back etc too just to be on the safe side and to make sure he isn't moving because he is in pain.