unfortunately, I had the same problem with my two kids. the only way we could get them to stop coming into our room when there was bad weather was to try and reinforce the fact that they were "big" kids, and that they shouldn't be worried about it anymore.

My twelve year old son just loves reading about different types of weather and watching the weather reports on TV but he has a great fear of severe thunderstorms, and sometimes just rain alone. He is afraid that one day tornado will kill us. I have taken him to the professional also but nothing cures it.
Please help me. What shall I do?

It's just a phase in development.
The difference between my child is 18 months. They are, in a period of 4-6 years, regularly come into our bed at night, regardless of the weather.
We would just tucked them and continued to sleep.
They have ceased by themselves.

Does your son have an older brother? I know when my sister use to go into my parents room during storms my parents started to put our beds together and that seemed to help.