One thing you can do is mount up and instantly demand a step backwards. It works with my boy. If they're being particularly antsy you can also bring their head around towards you because the only thing they can do then is go in circles - nowhere near as dangerous as the potential for them to go galloping off when you've only got one foot in a stirrup!
Something else that may work is to get on and get straight back off again, and keep doing it until he stands nicely - and then you don't ride, because the aim of even getting on was to get him to stand for you to mount him, and he gave you that, so he needs to be rewarded for it.
Another tactic that one of Western Australia's top horsemen uses, is to use the horse as an armchair - he will literally read a book and have a beer on horseback! In the round pen of course. It's incredibly effective. This horseman can start a horse and have it calm and settled under saddle with CHAOS going on around it in 4 days. Mind you he spends 7-8 hours a day in the saddle and will spend as long as it takes to get what he wants out of the horse.
Not standing for mounting is actually a very common vice with older schoolmaster types. They know their job so well that they know that when you get on they have to work and so they do so without even being asked. What YOU need to do, is break that connection between mounting and moving forwards. That's where the backing and the 'armchair tactic' are useful. It may take some time, but it works.
Also, if you can, having someone hold him for you to mount is very helpful. Very very few pro horse trainers go it alone for the first few rides. Most of the GOOD ones don't even ask the horse to move the first few times they mount up. It's scary enough to have a person up there without the added pressure of trying to find your balance now that your weight's going in different places!
Oh, also - do EVERYTHING 3 times. Every session. Even if it's an instant of standing still, if it's an improvement, you praise it. On the 3rd session, be prepared to settle for a little less. 3rd time anything is tricky with horses. I don't know why - it just IS.