Ok. Take a breath.
As I see it you were in a relationship that appeared to be financially 'equal' (i.e. neither depended on the other). This dynamic hid some facets of your DP's character. This is no one's 'fault', we often don't see people for who they are when things are going well. Why should we when the issues causing conflict/distress don't arise?
But now, you are seeing a side of him that wasn't apparent when you didn't need to 'ask him for money' (as he would see it). He's selfish and sees things in the view of what's in it for him. And he's very unlikely to change. Selfish people rarely do.
This is much bigger than a mere £15, or even whether or not he'll help finance your mat leave. (hint; he either won't, he'll hold that he did over your head for eternity, or he will make you beg for every penny) It's indicative of how the rest of your life will be. Even if/when you return to work and again become financially independent there will always arise situations where income needs to be pooled for some reason. Home purchase, children's education, cars, careers, even retirement planning. And his basic philosophy will be "How will this benefit me" not "How will this benefit us". Every decision will be an uphill battle. Is this what you really want?
As far as a solution, it's entirely up to you. You can choose to live the rest of your life with someone who will never give due consideration to your needs. If you choose that, then you will need to make up your mind that you will have to facilitate your (and your child's) needs completely out of your own pocket probably 80% of the time. He will consider childcare costs yours because 'they're enabling you to go back to work'. He won't see that childcare enables HIM to work, too. He will see all the accoutrements of babyhood as yours too,. After all, isn't that what mat pay is for, to spend on baby things? (Answer; no).
If this is what you choose, then I'd separate finances completely. Because chances are that he will view his purchases from a joint account as 'necessary' and your purchases as 'optional' as you've just seen.
Or you can leave and begin to set up a new life where you aren't having to justify or account for spending money on items you need or want.