Stop worrying about somebody stealing your precious idea. Your idea is worthless, and its not very good.
Hold on. I am not trying to be nasty. I will tell you a little storythe much abbreviated version.
How do you know his idea is worthless... or that it is not very good.
The op never specified what the idea was.
my advice would be work out a plan on how to provide POC of your idea.
or possibly create a detailed plan outlining the idea.
Work on networking in the community if your project requires coding.
You have lots of options open to you. I guess you should do some research and document your idea would be step 1,
Good luck