My doctor asked me for coffee. What to do?

I am a fairly new health care provider, and go to a medical doctor for treatment of a medical condition. I have been to about 5-6 visits with him over a period of about 6 months, but have not been in to see him since fall 2011. He was always very friendly...let me call him by his first name, sent me emails to see how I'm doing through his work account, and checked in frequently via phone. I figured he was just really nice, and was more informal with me because we are both health care providers. As I said, my last appointment with him was back in fall of 2011. Yesterday he emails me to see how I'm doing, and asks when am I going to stop by and visit. He is a part-time practitioner and a part-time medical school instructor. He is married, or at least wear a wedding ring. I am in a relationship, but I have never discussed anything personal with him, including my status. The conversation always centers around my medical condition and how my treatment is going which is where I like it. Today he emails me and asks me to let me know next time I am in the area and he'll buy me a coffee. I don't know if this is normal networking and I am just imagining things, or if he is out of line. What do you think?