Well you should know smart people eat other smart people. Its a game of getting well known like Nobel Prize winner. In research & development there is always people who are willing to "Borrow" your idea because it is easy. No hard work require, you just need to provide the logic explanation.. plus he gets the name not the one doing the job.
My advice to you and do remember this, In an any ingrident or research there is ALWAYS a small secrete which make the whole thing works. Give him the basic but not that special ingredient as a FAILSAFE. In case he steal it from you, it will not work. Only you know. Until you become the senior, best keep your findings to yourself for your future.

Fair enough, but I guess the thing is in general, academic research is meant to be in public domain. Openness and sharing is part of what makes academia different from industry. Take the necessary measures to make sure u r credited for your work, but beyond that, there's really no point to be too hard up about it. Ideas come and go by the caseloads.

As a junior researcher/student, there's only so much that u can do. Keep lab books/logs up to date, disseminate info that u want to make public knowledge (even if its just telling colleagues and PI), don't be stingy with ur skills and know-how but be smart on how u pass it on. The most important thing really is pre-emptive - choosing the right research supervisor/group to get involved in. I think this is far more important when ur concerned about any research isu. The right ppl makes all the difference.

let it go, be the bigger man. just be wary of this guy from now on.