I can see both sides, to be honest.
It does sound a bit brutal and unfeeling of your husband, especially the way you've expressed it here. Does he have empathy for the fact that your son has just lost his dad?
OTOH it's not your husband's fault or responsibility that your ex died, nor that your ex's wider family don't contribute to your son's birthdays. Isn't that something you would want to tackle with them directly? (I'm thinking specifically about maintaining lines of communication with grandparents here.)
Also, I'm not sure that it's helpful comparing one set of family circumstances to another. At one level, yes, it sucks that your ex's family circumstances don't allow for the same level of financial generosity towards your children as your husband's family's does towards his. I get it, it's shit. But it's also a reality, and more specifically the reality of a blended family situation. Not everyone's circumstances are the same. And there are things you can do to adjust/counteract that, but also some things just have to be different because they are different.
You've not put much in your post about your relationship with your ex. But in a way, throwing more into the birthday pot might risk looking like you're trying to overcompensate for his dad dying by spending more on birthday presents.
But maybe it's not an 'extra gifts' situation, in a wider sense. I wonder if the tactic might be to tackle the loss of his father separately, differently.
You can't assuage grief with gifts. I wish we could, but we can't.