I am communicating with a person in the U.S. who is about 40, and holds a university professor position, while I am a prospective student. Usually he would address me with 'Hi, MyFirstName!', and he has signed in many ways - from the official 4-line signature to simply his first name. Does it mean that he expects me to address him with only first name, too? Or I should better continue with the formal 'Dear Mr. FamilyName'? Is it ok to use 'Mr.' to address a professor at all, or 'Prof.' is better?

Thanks in advance.
Hello Bubr,

I will pretend that his name is Bill Smith for these examples.

I think that you should use Dear Prof. Smith for now,
and wait to let him tell you if he wants you to use Dear Bill.

I say this because it could never be rude to use Dear Prof. Smith,
but could possibly create small problems to use Dear Bill or Hi Bill.

If he wants you to be less formal he will tell you, "Just call me Bill".

Don't worry, Americans are very comfortable about saying things like that,
so he will tell you if he wants you to be less formal.

Until then, a little show of respect can't hurt. Emotion: smile

Good Luck!!

Thank you Trellis!