You may not know him well but your son will. This guy IS your sons family (if not yours) and he has been a big part of your sons family since your son was born. This is really not your choice. You are making a big deal out of something that really isn't. My kids call my moms husband "Grandpa W" and sometimes use his first name but it is THEIR choice because it is THEIR relationship. Not mine. and they were in elementary school when my mom married him (and she had two husbands before him, both grandpa). They are not confused, no one is devalued. There is enough love for everyone. It sounds to me like you are trying to create space or maintain control. I say just let it go. Clearly your husband does not have a problem with "Pop". and if anyone has a right to have a problem with it is him.
I totally agree with lilyka - don't expend any more mental energy on this, just let it go.