Thanks for your question.
If things go well the boss will find out anyway, so you may as well assume you will do well and have a conversation with him now. Now we don't know enough about your idea to really advise well enough, but in general I'd try and position it as a 'weekend hobby', and as something he/she shouldn't be worried about.

Also, I really wouldn't worry about the boss doing what you do just because they can. I'm guessing anyone connected with you on social media or networking events could do so too. People are way too busy with their own stuff to allocate enough time and resources to do what you do, nor are they as advanced in the business as you have been, given you've spent valuable time gaining key insights that they couldn't never gain from hacking a version of your business.
Hope this helps. Good luck!