Hi Lyla
Long distance relationships can be tough even at the best of times.
I've never been exactly in that situation but I did smoke pot for nearly 30 years - from your post it does sound like your BF is in deep.
The only thing I can suggest is you try and open up some communication. If you've tried that, and he's not communicative or he dismisses your concerns, maybe you need to think about what you're getting out of this relationship?
D