What are your financial arrangements now? I think rather than thinking about “when the baby…” scenarios it’s more helpful to organise your finances in terms of retirement and legacy (your bloke would have a nervous breakdown with me)
Personally I think keeping separate finances as a married couple is not advisable. It’s an illusion because in the eyes of the law the assets are joint and where those divisions are drawn becomes clear in divorce.
If you think how you will cope in old age, the obvious thing is pensions. How much are you contributing? (And consider how to prioritise pensions in different periods of life (contributing more in good times, and how to maintain some level of contribution during periods of leave/illness/unemployment)
Think about disasters - in the event of death the surviving spouse is better off if bank accounts and bills are in both names and there is a will instead of going through probate. Even though the entitlement is the same one way is much easier and doesn’t leave the surviving partner stuck with frozen accounts for a while.
Having a clear idea of how much income you are generating, a single rough figure for your costs (housing, utilities, groceries, transport, clothing, necessities) and what the excess is. Then make decisions about how that should be used - what % in savings ( for emergencies, for old age) and what % for life satisfaction (holidays, fun, hobbies, frivolous)
One method is to set up your bank account to feed into other bank accounts. Wages paid into account A, dd (immediately after payday) bills, mortgage, etc. dd to savings accounts, and then put a budgeted amount into B aGroceries (and other stuff) account, C a variable costs (tax, insurance, health) account, and D&E pocket money into an account for each of you.
Obviously this assumes both people are fair and trustworthy.
But using this model, and the principle that in an equal partnership both are entitled to equal access to the money and to rest, maternity leave could mean there is less money in Pot A - more needed in pot B (nappies and baby clothes), higher utility bills, so D&G both have to be cut back.
And when mum goes back to work, Pot A has more money coming in, but also now a whopping childcare bill coming out, grocery costs same, utility bills might drop slightly. D&G might still be low.
Compare that to a very common split costs situation (so depressingly common on MN) where each pay half in bills. She has salary loss, so supplements with savings, and cuts fun spending and then shoulders the childcare costs when returning to work. Meanwhile he is building wealth with savings, pension, and likely increased earnings if he is promoted, and can afford gym membership or an expensive cycling hobby and rugby weekends away with his mates.
If you set up your joint finances wisely at the start you can weather all sorts of problems along the road.
If you take the immediate issues of the baby out of the discussion, would he be willing to talk about how to structure your joint finances, where the money is currently going, and what long term savings and investments are in place. Talking about the right now might feel more tangible than the what ifs.