I was recently official witness to a woman being christened in her 50s. Baby's age is nonsense.
You're not trying to control this, you're trying to meet half way. He isn't trying, so it's clear to me who is controlling.
I'm concerned your eldest is living with someone who doesn't view him as part of the family.
I'm concerned you're not getting a say in fundamental decisions. That is WRONG and abusive.
I'm also concerned that you are isolated from people outside your partners family.
Phone the vicar and explain the situation. Be very clear that you do not want your baby christened on this date.
Phone Women's Aid and ask for some advice about back up plans and freedom programme.
Get a 'go bag' ready - so you can leave at a moment's notice and have essentials. Including baby's birth certificate if you've already registered baby.
If you haven't registered baby yet, then go and do that by yourself. Give baby the name & spelling YOU want, and make sure your surname is included in baby's surname. I'd actually consider either putting your partner's surname as a middle name or leave it off completely.
You need to get away from this man. Don't bother trying to understand him, focus instead on where you are going to live & how you are going to get away.