Are you certain that he also loved the name for five years as well? Might it just be that he was perfectly happy with it when it was all theoretical, but now that it's all becoming real, he's started to have doubts?
Not the same thing exactly, but when you get people promising to give hundreds of thousands to everybody they know IF they win the lottery, but if they actually do win it, they don't always feel quite the same....
I’m an absolute loss to understand why men think they have any input into this anyway 🙄
When they can grow a baby, they can have an opinion
Do you think the same about women who choose to be SAHMs who think they have any input into the house they buy/rent or how the household budget is spent, if all of it is earned by their husbands? Even when you're on ML - and later, if you work PT (likely because of your baby), is it OK if he says "Well, when you go out to work FT, then you can enjoy the benefits of a higher income" ? Especially when you've already made it very clear to him that it's really your baby and he's just there as a bit of a helper; not an equal parent at all.
Be careful what you wish for: claiming priority rights in all decisions to do with your child after they are born can very soon turn into your responsibility for everything child-based as well. If you're married to a certain kind of man, he will cheerfully make sure that all of it ends up being down to you to do, as you are the primary parent.
Of course, it's completely different in cases of abuse and deadbeat dads who don't want to know and walk out on you; but in a loving, committed marriage/partnership, you're a team - for the nice bits and the considerably less nice bits.
As PP very well said, comparing it to consent to sex, if one of you doesn't want it - be it sex or a name for your joint baby - it doesn't happen.