It does not matter. You can call him either. You can even call a full professor as "Dr." and it should be fine. There is nothing wrong in calling an assistant professor as "Prof." When I was a student in IITD, we used to always call our teachers as "Sir" so this was not even a question for us. When I went to Minnesota, I used to call my professors (irrespective of their designation) as "Prof." I don't even know the designation of some of my teachers in Minnesota. It would be really a hard task to remember the designation of each and every teacher and change accordingly. So, in short, it does not matter; you can use Prof. for any level of professor.