Hey,
Although I have been shadowing a D.O. for a few months, I can completely relate to your situation. I know this may sound obvious, but I think that the questions you ask him should honestly be things that you want to know; which I am sure will naturally come to you during the course of your experience tomorrow so relax and be yourself! That being said, I would definitely ask the general "why did you want to become a doctor, what do you like/dislike the most, etc?" type of questions. I also asked what differences or similarities arose when working with DOs and MDs. Also, the part that stands out the most from the information you have provided about the doctor that you are shadowing is his role as the president. I am sure that you can ask him several questions regarding this subject, i.e. the similarities and differences between Canadian and U.S. medical schools, how he arrived as such a position, etc. You can even ask him questions about any unique cases or patients that he has dealt with.
Lastly, as far as paying for lunch, I suppose that it would be okay if you offered because not doing so can give him the impression that you automatically assumed he would pay. Besides, nothing bad can come out of offering and then actually paying or offering and then having him decline. Anyways, hope this helps and good luck on Monday!