I'm an undergraduate student in a research group. I'm a little bit confused about how can I call my professor appropriately. The graduate students in the group just call him by his name, but I think it would be a little bit rude for me to do that. I want to be more polite, but I also feel like it's a bit weird if I add 'Dr./'Prof.' every time I'm trying to call him. Any suggestions about what I could do?