I recently contacted a potential PhD supervisor expressing my interest in working with him. I attached all application materials (personal statement, CV, etc), as he requested on his website. I received a prompt reply that he is accepting graduate students for the upcoming semester and to let him know if I have any questions. How should I follow up with him, and what sort of questions are appropriate to ask at this stage? I submitted my formal application to the program soon after receiving his reply.
As suggested by @Austin Henley, take a personal approach (phone/Skype is best). PIs want people who are passionate and who fit well. Plus, a bit of ego stroking never hurts. The candidate who takes initiative to call/Skype demonstrates interest and initiative, and a little bit of ego-stroking that can pay dividends. At that point, you become a person, not a piece of paper.