I second everything said on here, but make sure your farrier knows about how your horse is, before he gets there, i.e. Tell him on the phone and ask him if he is happy to attempt the horse.
My gelding used to do that also, it only took a few falls on his face to realize well hey, no I will not hold up your 700kg of lazy. He was also bad with one back leg, he would shift and sway his leg back and forth unless he felt it was held securely.
Just another thought from my own experience as my farrier had knee surgery a while back so I make sure to look after him real well... Tell your farrier, that if he does get under the horses back legs and manages to do them, that you would rather he did the telling off if the horse shifts as opposed to you doing it at the head of the horse and horse getting a fright and stomping all over the farrier who is in an awkward position.
My farrier in particular likes this arrangement for the back legs because he has had several horses that have shifted, or even done something the owner didn't like, like shift its head, and the owner has belted the horse, which has responded by jumping and moving, while the farrier is in a potentially dangerous position with the horses back legs. And because the farrier is in such close proximity to two lethal weapons, he/she will be able to feel every movement your horse makes and react accordingly.
Good luck, my horse was exactly the same to start with, I even went to the extent of wearing my helmet every time I tried to pick up his back hooves, just in case.
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms