I think that there are a lot of societal influences with regard to how to begin addressing a Japanese man with his first names. Please put the societal issue aside and just consider the linguistics of this scenario.
- Both the Japanese and American are speaking in Japanese.
- Both the Japanese and American are men.
- In formal and informal settings, a Japanese addresses me using my first name.
- I address the Japanese using his last name.
- During the first few weeks / months of getting to know him, he uses my first name and I use his last name. I can't remember how it happened, but I've been speaking to some Japanese men using their first names for awhile.
- I prefer using his first name if I consider him a close friend and if he has no problem with it. But, in the presence of another Japanese person, I would never say his first name.
When I think the time is right (which is not this question), how would I express this thought in Japanese? The context is an American saying it to a Japanese.
Hey Tanaka-san. Is it cool If I start using your first names?
I would say something like
Is that ok? I am sure it sounds very unnatural, but even asking to use his first name is unnatural. So, what should I say?
By the way, my observations are that, even when best of buddies, Japanese men always address each other using last names. And, the norms for addressing Japanese women with first names are completely different.