Taller people generally have the advantage of reach. When this is the case, the only thing that will help you is footwork. You have to be able to move inside of their range. Once you get inside, work their body, make use of hooks, knees, elbows and uppercuts and make angles on him - don't stand in one place. Think about tyson in his prime. He was short for a heavyweight. He used superior footwork to get inside of everyone. Now, watch him today - he can't get inside anymore and he gets killed. footwork is key.

darkfist has the right idea, but I disagree that once you take his 45 and attack that he will not regain balance. There are no absolutes in fighting. Ideally, you don't want him to regain, but realistically, he very well may. you can't always retain contact either.