I was really scared of storms when I was little. It didn't go away until I was around 10--maybe older, but I remember a couple things helping:
- Watching the lightning with my mom. Sit with your son and look out the window. Watch how the lightning makes everything show up in the dark. Show him that he can see his sandbox, or the fire hydrant or whatever, with every flash of lightning.
- Understanding how thunder is related to lightning. After every lightning flash, count to five. See how many times you can count to 5 before the thunder. (1-one thousand,2-one thousand,3-one thousand,4- ,5- ; 2-one thousand,2- ,3- ,
4- ,5- ...) Sound travels about a mile every five seconds. For every time you can count to five, the lightning was one mile away. You can tell if the storm is getting closer, or farther away. You can tell him things like, "That lightning was 3 miles away; that's as far away as... McDonald's, the park, big brother's school..." you get the picture. Celebrate the really loud thunders. "Yay, that was a good one!" (It's okay to admit that one "snuck up" on you, and you were startled too! If he can see you are calm, he'll pick up on it.)
Hope this helps.