Well... your stepdad is not his grandfather.  And... he probably hears what you call him so that is the association for him. 

My YSD took to calling her mom by her first name when she was a teen.. drove her mother crazy.. called my DH by his name occasionally too..lol.

I was always "esmod" to them... never got called mom.. introduced as "my stepmom esmod"

If you get the feeling he is doing it as a lack of respect thing.. maybe curb it.  But I think people stand on less ceremony with Sir and Maam etc.. these days.

If he is a member of your family then he follows the customs of your family lest he thinks of himself as a visitor.

I think you were wrong. Your SS should get to choose whatever respectful name he wants for you and your family. It doesn't matter if they treat him like their grandson. He isn't, and your SS wants to make that distinction.

Another thought.. it really was only an issue with the YSD since we didn't have the OSD with us much when we saw my father.  Now, my dad is a bit of an odd duck.  He never really took the girls in as if they were his granddaughters but they did get a few things from him.. but it was more along the lines of what you might gift to a child of a close friend.  In any case, my younger sd wanted to know what she should call him.. he said "the Colonel".. he was a colonel in the army.  He didn't mean it super stuffy.. but he obviously didn't want her calling him gramps..lol. Even with my brother's bio son.. my dad isn't a typical grand dad type and doesn't act like one.

I think that SS9 should call your stepdad a name that they are BOTH comfortable with (like if SS didn't want to call him grandpa.. but stepdad DID.. it shouldn't be forced).  I think that either Mr. (sdad's last name) or some other moniker that your Sdad finds appropriate is what the boy should call hiim.. not "tom".

One way to find out if it's just an issue of wanting to call adults by their names is to give him the choice of Grandpa or Mr. ______ (either his first name or his last name).  I don't draw much of a distinction between your mom and your stepdad - neither is the grandparent of your SS - so it could be hurtful to your stepdad to be singled out to be called by his first name.  My SO's kids don't use "grandparent" names to refer to my parents, but they also don't call them by just their first names.  It's Miss firstname and Mr. firstname.  To me that's a sign of respect, and SO is a southern boy who never called adults by a first name without a title and wouldn't dream of letting his kids do so.

I brought my kids up to say Grandma first name, Grandpa first name.  Both sets of parents were divorced and it just made it easier to keep everyone straight.  When they were hanging out with them one on one, they just said grandma/grandpa.  

I always wonder where a kid gets that idea in his head, a major change like that.  I'd be curious to have that conversatoin and see what's been planted and what's been pondered.   

A 9 year old should call an adult Mrs. or Mr. First Name... your SS should be given a choice, “Mr. Firstname or Grandpa”.

Honestly I wouldn’t force it. You don’t call him dad so how is it different them him not calling him grandpa. You have your reasons and it’s not because of anything negative.

SO’s youngest was SUPER excited the last time he saw my dad because he asked if he could call him pawpaw like my nephew does and he was told yes. However, my mom who he sees more is still called by her first name. I’d say they have a closer bond with my mom if I had to say anything. I think the only time they see my dad though they see him with my nephew and hear him called pawpaw. My mom though they see all the time and hear her called her first name by my boyfriend so the youngest picked up on it and is following it.

If you want him to change then you need to change too and lead by example but don’t get onto him about it. As long as he’s using a title that is respectful and yes first names can be.

SS  is nine. You and SO get to guide him through social norms. so think carefully about  what you want. 

You have stated that you call your SDad by his first name. But, what do you call him when you are taking to SS about him. If you use his first name, you are telling SS that he is not related to him even as a step grandfather.

When you use the word outwardly when you say your Sdad would not outwardly say he cares, you know that it would bother him. And if it bothers your Sdad, it will affect not only SS's relationship with him, but yours too. And what about your mom? How will she feel about her being called Grandma and her husband called by his first name?

Also think of your future children. Is Sdad going to be worthy of a grandparent name then? Or will be be first name, the guy who lives with Grandma?

Think carefully about what you want as this decision may affect both SS's and your future relationship with your Sdad. I say this as first name, who when the Grands call me first name,  feel like a mixture of grandpa's live in and a Victorian lower servant (not the wife of twenty years).

I tried to get them to call my parents Grandpa Jerry and Grandpa Karen but they see them so infrequently they kept forgetting their names entirely!  This summer, SS9 said to my dad and mom "You look too young to be called grandma and grandpa, so I like to call you Aunt Karen and Uncle Jerry"  They loved that!!! hahaha

It could be a power issue. An Equalizer if you will. SS7then6 started calling BM by the name we call her. Its the nickname she had her Ex Step daughter and godchildren call her. When we first heard it we asked him why and he said it was because he wanted to so we didn’t push. When he said it to her face, she freaked out on DH claiming we were making him call her that “because SS said you call me that.” Truth is, he is a mini-husband-her golden child and he runs that house. She caters to him out of fear of not being his favorite parent so she plays the bestfriend card. DH told her the truth. He told her that she lets him believe he is her equal so he wants to address her the way her other equals address her-by her name. 

I grew up calling my maternal grandparents by their first names (sort of - one was a nickname but that's what everyone called my grandmother, so as good as a first name). I called my paternal grandparents Grandaddy and a made up nickname sort of like her first name. My ex called his parents by their names and my son calls all of his grandparents by their names. I am almost 50 so not exactly a millenial. 

I sometimes call my mother by her name - I know she doesn't like it. But it gets her attention. She's very very good at ignoring people who call her mom. She doesn't particularly like being called 'first name' by my son either. But it's stuck now. My step kids call my parents by their first name. I don't know what my son will call my partner's mother and stepmother. He's only met them once so far. 

Hopefully when I have grandkids and step grandkids - they'll just call me and my partner by our first names. I'm happiest with that. I don't think it's disrespectful or a power play. Or maybe they could call me by my rugby nickname. 

OP - I think you should talk to your stepdad and ask him what he would like to be called directly and influence that. He may not actually care. It's the relationship and not the name that matters.