When I talk to Nathan's parents, I start a conversation with "Hi Dad, hi mom!!"  And they look happy to talk to me.  But how about Nathan? 

A few days ago, we called my father to talk about an website of his company because we are in charge of it.  After we hung up, Nathan asked me what he should call my dad.  I though he should call my dad "otosan" which is father in Japanese.  Nathan thinks my dad doesn't want Nathan to call him "otosan" because Nathan's age is not young enough to be his son.  Currently, Nathan calls my dad "Sato-san" which is Mr. Sato in Japanese.

I hadn't minded so much about how Nathan should call my dad until Nathan asked about that.  I think what ever Nathan likes, but I want to help Nathan to talk to him naturally, if Nathan wants to talk to my dad naturally.

What should Nathan call my dad?

you are 100% sure i should call him dad, but i am sure he would look shocked if i did it.

he always looks shocked when i talk to him.  it doesn't matter which language i use. 

I aｇree with Rice.

"Oto-san" is natural.
the age is doesn't problem.

Nathan should continue call to him "oto-san".

すぐに　ふたりとも　その　よびかたに　なれます。

It is important to continue.

but i don't call him anything now.  just "Hello" or "Hello, it's Nathan" so, it's not continuing.  i think i need to start something because I called his shop a few days ago.  i asked the staff if he was available and they transferred me without asking who i was.  when he picked up the phone i didn't know what to say so i said "it's Nathan" and stopped.

I think the reason why Nathan feels awkward to call my dad "otosan" and he thinks my dad doesn't want him to call "otosan" is because their ages are not like a son and a dad.  I mean my dad is 55 years old and Nathan is 41 yeas old, so 14 yeas old difference, which means my dad too young to have a son who is Nathan's age, and also the age difference between them are the same as Nathan and me.

I'm now sure the reason is only their age difference, but I think this is a part of the reason.

If you were my dad, how would you feel?

ok everyone, i was trying to be polite here, but i give up.

i don't know what he wants me to call him because i don't know if he wants me to call him anything.

we don't talk much.  now it's because M & i live in the US now, but  also when we were in Japan because of the language problem.  i think it makes him nervous or annoyed to try to have to understand my poor Japanese.  sometimes he can understand and someimes he can't.  that is a different discussion that goes back to the topic of Japanese people not having experience with listening to non-native speakers using their language, but it is part of the problem.  he will often hand the phone to someone else to "deal with" me.

as for what he wants me to call him, i don't think he considers me a son, so dad is not really going to work. 

he has been nice to me and us, taken care of us, helped us when we asked, but he is pretty much unreadable.  his poker face is better than Matsui's and it is alway on...  just look at our wedding pictures for proof.