Groundwork advice (mounting block)?

Ok, so I've had my 7 year old OTTB for about three months now. We have been riding english and jumping a lot. He is over 16 hands, making it hard for me to get on him without a mounting block. He likes to swing his rear end around, away from the block, or back up. The only success I have with the mounting block is when someone holds him for me. What training exercises can I do to improve this? Oh and also, when I try to do my two- point at a trot or canter, he breaks into a gallop. I'm thinking he does it out of habit from racing, but is there anything I can do to correct this other than circles and pulling back? Other than these two things, he is a perfect gentleman. All advice is appreciated!