For some reason, your kitty has been conditioned to think sneezing is a bad thing. Your cat thinks something bad will happen to him when he hears a sneeze. In his past, a sneeze or a sound similar to a sneeze, could have conditioned him in this way.
You need to reconditioned him. To do this, you will need to keep his favorite treats throughout the house. When you feel like you're going to sneeze, run to one of the treat bags/containers and open it. Hold the bag out in front of you so the cat smells and sees the treats first. When he calmly responds, give him a few treats. Obviously, reconditioning for your specific issue will take a while, unless you sneeze everyday. (If you do have allergies and sneeze quite often, be careful not to over-feed on treats. They can cause a cat to gain unhealthy weight.)
After your cat calmly approaches, consistently, over a period of time, give the treats every other time you sneeze...then, every third time you sneeze. Soon, you will have a calm and happy cat and will no longer need to give the treats. Stick with it and be consistent. I wish you the best!