Potential PhD supervisor expressed interest, but had a short response. What do I reply? I emailed a potential supervisor at a US university recently. He replied promptly, cc'ing the graduate school dean in the email and saying I had a very good idea but then suggesting another research question that needs to be answered first. 

I saw that no one has done it yet (nothing published, as far as I know), and I then asked if his lab is currently tackling the problem, and he simply responded:


  We are not working on it yet


I have asked the graduate school dean several questions after that, but I have not responded to the potential supervisor yet. How should I continue our correspondence? I'm really interested in the new research problem, but I don't know how to respond to him.
I think he is busy and just responded in the quickest way possible to your query. If you are interested in it, respond that you are and would like to take it up. More importantly, respond that you want to take it up under his supervision and will be following up with your formal application. 

If you come up with any deep thoughts on the problem or partial solutions, you can (now or later) communicate that you have progress and maybe share some of that. Since you don't really know much about him, I wouldn't lay everything out early on, however. Wait until you can assess one another first. 

If you read other questions on this site note that some of them are about unethical behavior by supervisors. I have no reason to even suspect this is the case here (and since it is rare, I doubt it). Others questions are about misunderstandings. If you are familiar with the game Poker, don't show all your cards early.