For example, his name is Taro Okamoto. Taro is his first name and Okamoto is his family name.
In formal settings, for example, you're on business, in a company, or in a class at school, you should call him "Okamoto-san."
"Okamoto", "Taro", "Taro-san", "Taro-chan", "Taro-kun" are not appropriate.
"Okamoto" and "Taro" are even rude and offensive. No matter how close you and Mr.Okamoto's relationship is, you should call him "Okamoto-san" in a formal setting. It is a matter of Japanese grammar, the Keigo-system.
It's more like in English you should call him "Mr. Okamoto" in a formal setting.
In informal settings, for example, in a private party, or in a conversation between friends, you can call him:
(The higher the number, the closer your relationship would be.)
- Okamocchan or Okamocchi
- Tarocchi or Tarochan
First, you should start calling him "Okamoto-san" and I think it's safer to ask him the permission about what number you may call him.
Hope this helps!