I have been on both sides of this issue.
Some background: Exh is DS#1 father. I was his 2nd marriage, he was my 1st. He had 2 children from a previous marriage. He was horrible about paying his support unless it was garnished or I paid it.
We got divorced, I remarried. He has never paid me any
CS. Not a dime. I filed once, he threatened me, I withdrew. I may change my mind at some point, but the minimal amt of support he would be ordered to pay to DS may not be worth the fight. My DH supports DS#1, and DS#1 paternal grandparents have remained a significant part of his life, and have bought him clothes, food, and have taken him traveling.
I just had my 2nd DS.
My exh girlfriend has just had a baby as well, so this is child #4 for exh.
So, just for fun, let's pretend exh was paying CS to both of his ex-wive's. (Ex wife #1 and me.) Let's say his total CS obligation was $100/month. So, if the court decided all of his children are "equal", then 1st wife is getting $67 and I'm getting $33. (I'm just picking easy numbers, because I'm lazy. I know this isn't realistic. Hell, I'd fall over from shock if exh gave me $33! lol) Now, if he went back to court and said, "Look, I have another baby I need to support," and they decided he could still only afford $100, the court may say, "Ok, $50 to wife #1, and $25 to wife #2, and the other $25 to help support your new ds." If that happened, I would totally be ok with that because I believe that none of his children with three different women are more important than any of his other children.
There are many factors that affect my opinion, such as I know exh makes almost no $ (so it's not like he's living it up on his yacht while I struggle), and new DH and I are doing ok, although it would be great even to have that extra "$33" to go towards DS college savings, or new shoes, or whatever. Also, if I did get support (hahahaha, sorry, can't help it) and it was cut, it wouldn't affect me that bad, whereas I know his 1st ex-wife would really struggle more.
So, what's the point of this story? Just that it's all about the children. If exh had 4 children with 1 woman, and he could only use $100 a month to support them, then I think they should each receive their share. Having the children with different women shouldn't change that. Should he have had that many children he couldn't afford? Probably not, but they are all here, and there is no return policy.
This is just how I feel, and I know other people may not agree. I'm also well aware that there are about a million other factors that play into child support numbers, like amount of visitation and the mother's income, but I just wanted to make it simple to get my feelings across.
To the OP: It's great things are amicable. Don't let him use that against you. Your choice to stay home or his choice to remarry is not material to the obligation he has to his child. Just be matter-of-fact and let him know what his obligation should be to his child. It's not a favor. Just talk to him, and don't even bring up you staying home or his new marriage. It's just, "Hey, let's go through these numbers again."
Good luck and I hope it works out for you!