Could your partner be just overwhelmed by having do his kids on his own OP? If so then that might be temporary. It was in my un-blend, eventually.
It was my partner’s decision alone to unblend, driven by his ex wife relocating to an area I couldn’t move to (child in good school and we’re near has dad with amicable coparenting).
I didn’t blame my partner because had he not moved he’d have gone from being a 50-50 dad to an EoW dad. We only had bad choices. And I was very supportive at first, travelling to his home a lot with my child to carry on being stepmom of the year. But I was burned out by this and my therapist helped me to put myself first. So eventually I disengaged and let him do the vast majority of his parenting all by himself. I seized the opportunity to build a full new life for me.
My partner was then very overwhelmed and seemed about to drop out of our relationship. He’d won his 50-50 quite a while after his divorce and when I was already there to support him. After our unblend this was the first time he’d been fully responsible for managing the house, managing childcare, managing kids. He was much less of a partner to me at first. He was knackered. He passively expected me to do everything for our relationship. Like I say I did for a while but therapy directed me to drift off and disengage instead. As none of this was my choice I took advantage of my extra free time and energy to work out, take holidays with friends, start new hobbies.
Partner adapted himself to his new situation. He got a parenting coach, sorted out a whole lot of kid stuff and his own physical and mental health.
Now this took him a few months but I simply put the relationship into cruise control to wait for him to sort himself out or prepare for the exit.
But he’s doing well now. Because he’s much more capable as a parent and a person he can put the time into our relationship as well as single parenting. We have a nice time together and it’s not only me planning it all.
I still resist going out of my way to help him with his life and his kids. My test for whether I put effort in or not is would he do this for me. But also I did accept where he was in those months of overwhelm, without enabling him to continue by running his life for him like some sort of personal concierge - and believe me he tried to make that happen!