It would only be ok if you had not been his patient within the past 2 years. This is no more acceptable than me dating some of my adult students. I have had some incredibly attractive students around my age but it is my responsibility to ignore their flirtation and remain professional at all times. Same with any of my clients. In the past I have referred a few clients on because I have felt they have been attracted to me and I wouldn’t want to take the risk (not all I refer because I don’t feel they are all ‘a risk’). I would be really careful in this situation. The fact that he’s flirting with you at all tells me his ethics and professional boundaries are not great.