How do I reply to a professor's email who wants to chat with you? I contacted a potential advisor for a PhD program. He replied to me that he was not in a position to accept any PhD students at this time. But he also wrote to me: 


  "I'd also be happy to chat about the program and my work generally if helpful, but would realistically have to be in mid-January. Let me know if of interest." 


I am not sure what and how should I reply to him. I am interested in having a talk with him, but what should I ask?
He wants to know more about your background preparation and your research interests. He likely also wants to get a sense of how you would fit into his research program. 

However, he may also just be interested in grad students generally and wants to think about whether some other prof would be good to work with you if he can't. 

I expect that the chat will be entirely informal and give you a chance to look around the department and maybe meet a few people. I once asked a prof if he would be my advisor and he declined, but suggested another, who turned out to be a much better match. I don't think he actually introduced us (long time ago - poor memory) but think he just suggested I approach the other (more senior) prof.