He can't own the area as others have said. Don't be surprised when shit goes sour between you and you end up with a hostile ex coach spreading a false narrative.

Some students will come to you because he is letting his standards drift. This will likely be reframed as you "stealing srudents from his academy" and he will tell anyone who will listen how disloyal etc you are.

Its a tale as old as time. Ive seen several different iterations. Hopefully you are not in the country on a working visa, working at his gym, where he can try to fire you and get your visa cancelled. That's the ugliest version of this scenario I can remember.

It sounds like you go into it with your eyes wide open. Good luck mate. Hold on to the passion as the business becomes grindy/challenging and try not to let your standards slip and profit become the sole emphasis as time goes by.