OP, I don't know if this perspective might help, but a few months before I got married my grandmother died. I started to feel this massive regret about all the questions I'd never asked her while she was still alive, and I signed up to Ancestry UK and started researching my family tree. It was a much harder job than it would have been if she'd still been alive and I'd been able to ask her things.
One thing that struck me was that it was so much harder to trace my female ancestors than the male ones. If you have someone's name and you know when they died then their death record will say how old they are when they died and from that you can usually find their birth record and then you've got the names of their parents and can find the previous generation. But this doesn't work for women because their death record was always in their married name and so if I wasn't able to track down a marriage record to find their previous name I wouldn't be able to find the birth record and the trail would go cold.
In some family lines I was able to go back a really long way, but in some female lines I just got completely stuck and wasn't able to go back any further because I couldn't find their birth name.
I think this experience directly influenced my decision not to change my name when I got married, because I felt sad that so many of my female ancestors' histories have been lost, and I started to feel that changing my name would be like erasing the whole identity I had before I got married. I don't believe that my life began when I got married. I had a whole life and many achievements before that. And I also married a man from another country, so taking his name would also have felt like erasing my national identity, not just my personal one.
I know there's more of a trend for double barrelled surnames these days, but even if you do that, you can't just keep double barrelling and end up with a million surnames. We can't bear all the surnames of all our ancestors, so some selectivity is necessary. But I really do feel that if a baby is going to have his father's surname, it's really important for the mother to be able to include a given name from her family if she wants to, to preserve some of her own heritage.
My mum changed her name on marriage and thinks it's weird that I haven't. She also thinks "feminist" is a dirty word. But she also recently told me that when she and my dad got married she wanted to double barrel their surnames but my dad said no, and that when my brother was born she wanted to give him a second middle name from her side of the family and my dad said no. (For context, my brother's middle name is my dad's middle name and my paternal grandfather's first name, and the other name my mum wanted to use was her mother's maiden name and all the other boys on her side of the family have this as one of their middle names.) But for some reason, my dad just didn't want to and said no.
When we had this conversation, I asked her why on earth she just let him have his way. (My dad can be quite stubborn but he is generally a mild-mannered man and not scary or controlling.) She says she doesn't know. 32 years later and she still regrets it and doesn't know why she didn't stand her ground.
I think you need to very gently explain to your husband why this is so important to you. It's not even as if he has his heart set on a different middle name. Your son has his surname and you are entitled to have a name from your side of the family in there too.