Been there. In my case, my mum would have said nothing and if anything would have been like "oh men aren't in to that kind of thing, you're being princessy"
But if she had been the one to step in and do it, it would have annoyed my partner (now ex). Since he's a man that's something she would never do have done, Men are VIPs in her book. And honestly it probably would have made me feel even sadder. And my ex would have told me it was pathetic/childish etc.
You know the characters best but mum's position could be tricky here. Is she annoyed on your behalf and trying to fix it?
Every year it made me sad how my partner made no effort, and when other people made a big effort or even a small effort that meant a lot, it also highlighted the contrast - that my nearest and dearest couldn't be bothered. Or because birthdays weren't a big deal for him, he didn't get it despite being repeatedly told. The only world where he might have made a fuss at all would have been if I had no other loved ones at all. But I do so either he thought they had it covered, and he could opt out, or he hated having to compete with them......he was a contrary type.
Posters are suggesting you treat him the same. If i had decided not do anything for his, in retaliation, that would have made me feel sad like a a couple we NEVER marked any occasion or went out, but even worse, it also would not have bothered him in the slightest!
I mean I guess you could organise something fab for just you and the kids but that's not ideal either and they'll get the message that he's selfish/uncaring/miserable. (Which might be true)