I am so frustrated with Parker!!!!

The boy will just not listen to me when it comes to his fears! Yesterday, a man was mowing our neighbors yard. It really freaked Parker out to the point that Bobby had to take him for a drive until the man left. Today the man was back. We had just bought Parker a big plastic pool and some sand to make a sandbox. He was excited about playing in it until he saw the man again. We asked him if he was going to mow and he said he was going to weedeat the lawn. He wanted to know why we were asking and I told him about Parker and his new sandbox. (Parker is afraid of mechanical noises) He decided to wait to weedeat until tomorrow so Parker could play in his sandbox. That was so nice of him. I told Parker the man was going to leave and not mow but he won't believe me. He keeps plugging his ears with his fingers and he wont take them out. He won't even play in the sandbox now. It's so frustrating because his fear is so deep that he won't take my word for it. I don't know what to do with him. What can I do about this? He won't wear earphones. I'm just stumped.