Hi R.,
my 5 year old son, is terrified of heights! but he does suffer from autism and sensory issues, so it's hard for him, but we little by little encourage him to try doing things that might help him, and take it in his own pace. an example I can think of now, it's a few weeks ago we took him to seaworld, he was scared of going up in the shamu stadium but we took it slow, i didn't care what the people behind in the steps were saying....but he did it, even by himself he said he wanted to up in the 3 or 4 stories up playground..u know??? he tried it but he got scared, but i told him "if you want let's walk thru the side of the rope and we can get down in the slide, if you don't want to keep going" well he did, but he loved the slide (and that was really high) and he kept going up and down the stairs to go in the slide...but the most amazing thing was when I told him to go to a small rollercoaster they have for kids, he taught it was a train (he loves trains) and kept telling him it kinda was, we was eager to go in, he sat down excited he did hold on to me, but he loved it.
In my experience, w/kids w/difficulties try little by little, cause if you push them to do something that is scary for them, they will get more terrified and would never do it! Do what you feel is right for your son, remember he is little still, but have patience sooner or later he will overcome it...but he just like everyone, there are people that are scared of heights, dogs, etc...good luck!