2 added 13 characters in body
source | link

As mentioned in the other answer, it is always the best to ask the person how he prefers to be addressed.

In my opinion, first look at the etiquette in your country and university. In some cultures in the universities and companies, people call their workmates, professors, colleagues, etc. by their first name, regardless to their level of education or their place in the system. In this case, calling that professor with his first name may not be that much trouble and will be considered a polite manner.

If it is an etiquette to formally call people in your university, for instance Professor Enthusiastic Student; it may be a good idea to call your friend Professor Enthusiastic Student when you are in a group of students and other researcher, in the classes or in the lab meetings; and call him by his first name when you are alone and you have private meetings in which only you two are talking to each other.

I have the impression that this is the right way to address that person, because in the academic environment he is not considered your friend, he is your professor and you are his student, so calling him in a friendly manner may be considered impoliteness; also in your private meetings and sports club, you are fiends and he does not have any teaching or research role in this occasion, so calling him with professor title is probablyprobably considered inappropriate and unfriendly.

As mentioned in the other answer, it is always the best to ask the person how he prefers to be addressed.

In my opinion, first look at the etiquette in your country and university. In some cultures in the universities and companies, people call their workmates, professors, colleagues, etc. by their first name, regardless to their level of education or their place in the system. In this case, calling that professor with his first name may not be that much trouble and will be considered a polite manner.

If it is an etiquette to formally call people in your university, for instance Professor Enthusiastic Student; it may be a good idea to call your friend Professor Enthusiastic Student when you are in a group of students and other researcher, in the classes or in the lab meetings; and call him by his first name when you are alone and you have private meetings in which only you two are talking to each other.

I have the impression that this is the right way to address that person, because in the academic environment he is not considered your friend, he is your professor and you are his student, so calling him in a friendly manner may be considered impoliteness; also in your private meetings and sports club, you are fiends and he does not have any teaching or research role in this occasion, so calling him with professor title is probably inappropriate and unfriendly.

As mentioned in the other answer, it is always the best to ask the person how he prefers to be addressed.

In my opinion, first look at the etiquette in your country and university. In some cultures in the universities and companies, people call their workmates, professors, colleagues, etc. by their first name, regardless to their level of education or their place in the system. In this case, calling that professor with his first name may not be that much trouble and will be considered a polite manner.

If it is an etiquette to formally call people in your university, for instance Professor Enthusiastic Student; it may be a good idea to call your friend Professor Enthusiastic Student when you are in a group of students and other researcher, in the classes or in the lab meetings; and call him by his first name when you are alone and you have private meetings in which only you two are talking to each other.

I have the impression that this is the right way to address that person, because in the academic environment he is not considered your friend, he is your professor and you are his student, so calling him in a friendly manner may be considered impoliteness; also in your private meetings and sports club, you are fiends and he does not have any teaching or research role in this occasion, so calling him with professor title is probably considered inappropriate and unfriendly.

1
source | link

As mentioned in the other answer, it is always the best to ask the person how he prefers to be addressed.

In my opinion, first look at the etiquette in your country and university. In some cultures in the universities and companies, people call their workmates, professors, colleagues, etc. by their first name, regardless to their level of education or their place in the system. In this case, calling that professor with his first name may not be that much trouble and will be considered a polite manner.

If it is an etiquette to formally call people in your university, for instance Professor Enthusiastic Student; it may be a good idea to call your friend Professor Enthusiastic Student when you are in a group of students and other researcher, in the classes or in the lab meetings; and call him by his first name when you are alone and you have private meetings in which only you two are talking to each other.

I have the impression that this is the right way to address that person, because in the academic environment he is not considered your friend, he is your professor and you are his student, so calling him in a friendly manner may be considered impoliteness; also in your private meetings and sports club, you are fiends and he does not have any teaching or research role in this occasion, so calling him with professor title is probably inappropriate and unfriendly.