grandad drama! not sure where to post!

Not sure if this is the right group to post this in but I'm at my wits end! My mum and dad split up 13 years ago after my mum had an affair with her now Husband. Anyway my mum and step dad and I have never seen eye to eye until a few years ago when we started getting on. When I got pregnant my mum was a great support to me and is my best friend now! Before that it was always my dad and me against the world! We've always been close but since my son was born I refer to my stepdad as grandad and my dad ha now stopped talking to me over it. Saying he's not my stepdad and he's not my sons grandad. Which I'm not denying but he has a role as a grandad and my so will be around the other grandkids (on my stepdad side) calling him grandad. I have no issue with it but I don't know what to do now! It's been over a week since we spoke or he saw my son! I'm so upset by the whole thing I just want him to realise life is too short to worry about a name!!!