OK, so assuming you aren't planning to have DCs straight away, how about you put together a 2 year savings plan. Start with sitting down with your DP, working out what your living costs are and agreeing to split them 50/50 now - if that leaves him with very little after paying childsupport, tough, or you'll consider moving to a cheaper place so he can afford his 50% of your bills.
Any difference between what you pay now and what you would pay if you were only funding 50% of the bills, you save. And every other spare penny, you save, on the understanding you need enough saved to cover your 50% of bills for 1 year so that if you do go off on maternity leave and your DP hasn't had a pay rise in that time, then you can cover 'your share' and not need him to support you in that time.
What he spends the difference between his share of the bills and what he earns is his business, if he wants to over pay on childsupport that's his choice as much as if he chose to spend it on expensive shoes or mountain bikes or anything else. What you spend (or save) with what's left over from you paying your share is non of his business either.
obviously, if you get married this would need to be reviewed, but right now, his CS payments are his business, so long as they don't affect you.
If, when you look at it, he's not able to afford paying 50% of your bills, or that you aren't able to afford it, then you need to look again at your outgoings and arrange a lifestyle where you can do that.
Remember, once you've had a DC, you will have to factor in childcare payments if you are goign to return to work, this would be a 'household bill' for you and your DP, and would need splitting 50/50 (never let a man you live with treat childcare payments as 'your' responsibility to pay, they are a household bill) - assume around a grand a month if you are working full time. Before you start trying for a baby with this man, you will need to look at that and work out if he could afford that too along with his other financial commitments. Do not be a SAHM with a man who's previously walked out on another family, even if you could afford it. You need to keep your CV up to date just in case history repeats itself.