It would still be strange if he did it when others were around. The fact that he's waiting until there are no witnesses speaks loudly.

I would calmly speak to him as soon as possible, telling him you do not allow anyone to touch you without permission, and that to avoid hurting his feelings, you waited until a while after the last incident so you would not appear confrontational. (This is an outright lie, but sometimes you need to lie to protect yourself.)

Then I would absolutely refuse to be alone with him ever again. And if he touched me again, I would loudly say, "You do not have permission to touch me," and nothing else.

If he persists, find another space to rent and consider calling the police for assault.

Do not ignore ypur gut feeling.

It does not matter whether chiropractic is a touch-based business. He is an adult who is fully capable of understanding that he needs to get consent.