I really feel for your husband, I lost a best friend in a crash when I was younger and it screwed me up for a long, long time.
From what you've described, I think his grief has not been fully recognised - not by the family of his friend and I can see how damaging that would be.
Complicated grief is especially difficult to live with and I found the 10 year anniversary of my friends death incredibly difficult - the grief started building afresh around the 8th anniversary. It felt raw again at this point & I really struggled.
I'm not trying to compare your partners grief to mine, but I can see why he's struggling so much right now. At 10 years, I was no longer the person I was when my friend died, neither were our mutual friends. Our lives had all entered new chapters and moved on from the time we shared together...while my friend was...left behind. It was like losing them all over again.
With your husband's friend's family blaming him for the tragedy, his own guilt (and I imagine the fear that others must blame him too), it must have been difficult, if not impossible, for him to feel connected to his friend. Maintaining a connection with mine was absolutely central to grieving.
Without it...your partner must have been so lost all this time. I can absolutely see how naming your son after his friend is so important to him and honestly, I don't think I'd fight him on it.
Unlike (seemingly) everyone else here, I've never felt it a bad thing to name a child after someone who has died, although I understand why so many people feel differently.
It's a burden to the child only if you let it be. Don't project guilt, grief or expectation onto the child and they're not going to suffer. Unless you think your partner is going to struggle not to do this, then I don't believe it would be harmful to your son.
Is the name one that suits a nickname? Either a typical shortening or something similar (eg Margot as a nickname for Margaret)? Or even an endearment (Bear Grylls was called 'Bear' as a family nickname).
That way your son can be Name known as
I would also not mention your son on social media at all. It isn't necessary and puts his future privacy at risk (true for all children).
Lastly, I have immense empathy for the family but it wasn't your partner's fault. He was not responsible for his friend's actions or choices, or those of the driver. And just because that's painful to the family doesn't make it less true.
They cannot arbitrate how others who loved their son choose to remember him, especially when those people have suffered so greatly and for so long in their grief. It might also be that had they not directed their anger towards your partner all this time, he would not feel so strongly about naming his son after theirs.
Your partner's grief is as valid and worthy as theirs - it took my own experience and the kindness of my friend's family to teach me that, but it's true.
See if you can go with your partner's choice of name but use a nickname for day-to-day. Keeps the 'tribute' but also gives your son something new.