Mother-in-Laws Birthdays Boundaries

My husband doesn’t like my mom (he admits to that) and while she lives abroad, he only has to “handle” her over the phone on her birthday and two or three major annual holidays.
His mom, on the other hand, lives right near us and we often host her for weekends and holidays and he always love to make her feel like a queen by us, an experience that I end up participating in giving her through her favorite dishes, buying her flowers, fixing her guest room with beautiful gifts, etc.
On her birthdays, we always take her to a beautiful brunch, buy her a gift and card and call her first thing in the morning.

Yesterday was my mom’s birthday and I asked my husband to call her and wish her happy birthday.
He texted me around 11am that he has issues with signal and if he won’t be able to call yesterday, he will call today.
He didn’t end up calling, not yesterday and not today.
I reminded him today again and could see the anguish he is in over making a two-minutes call to her.

So while I cannot make him call my mom, I do wonder, why should I make him happy by over-delivering love to his mom when he cannot make me happy by calling my mom on her birthday?
It makes me happy because I know she is happy when he calls and I would like her to experience that on her birthday.

I know it sounds childish, but I see it as a boundary: You didn’t call my mom, that’s totally fine, I cannot force you, but the consequence is that I will not call your mom either.
Of course, it doesn’t feel good, but then again, boundaries never feel good, don’t they? I cannot find a loving way to be with it.