Business casual should be fine, but a suit would impress him more🙂
You should know your optometry news and also national news, he like to hear what you think about issues
Yes, the interview was very relaxed!!! I even wore sandals and jeans...haha! Dr. Johnston is very laid back so there's nothing to worry about. You'll find that the interview will be more like a conversation. All the best for you...good luck!
I only wore sandals and jeans because I interviewed just to see if I would get in (my first choice was SCCO). And yes, I did get in but hey I recommend that you look your best anyway, but don't go too all out i.e. a full suit. Business casual should be okay.