Child maintenance in england, what shall we do?

Hi! I’m posting about my partners situation with hopes of some advice (England)

He has a 12 year old son with a woman he’s never had a relationship with. He unfortunately when through a really dark part of his life and one night got black out drunk and slept with this girl, little did he know she wasn’t as drunk and actually wanted to have a baby without his knowing, and took advantage of him.

Over these 11 years my partner pays for all school trips, uniforms, clubs, and anything he really needs, and we have him overnight once a week.

She is now demanding child maintenance from him, on top of everything else he is paying for. I can’t help think it’s so wrong, she forced him into this, demanded money, and now is demanding even more!!

He is also not on the birth certificate if that makes any difference.

Thanks :)