Hello Atlantis8, I join with Cat Familiar in welcoming you here

"Radical acceptance" is a term that gets talked about here. A nutshell version is that RA is
not about "accepting that what he is doing is okay" or "allowing him to do wrong things" or "just letting him get away with it". RA is about: accepting that
he is doing what he is doing, instead of hoping he isn't doing it, or trying to believe he is someone fundamentally different, or any number of ways of not looking at what he is really doing.
RA can give us a foundation from which to move forward. Kind of like:
I wish he weren't so inconsistent, inconsiderate, and unpredictable. But: he is. That's how reality is. So what can I do with that, where can I go from here, now that I've accepted that he is who he is and does what he does?
You're seeing that he is consistently inconsistent. It would be nice if he weren't. But: (RA) he is. In a weird way, that can help you form a plan for going forward, and supporting your child in an appropriate discussion of reality.
One way that could look is having a chat with your kiddo: "Lots of families where Mom and Dad live separately have a schedule for the kids. You probably even have some friends who do that. But, not all families do. Our family is one where even though Mom and Dad live separately, we don't have a set schedule. What that means is that sometimes Dad calls to spend time with you, and sometimes he doesn't. When he calls you, let me know, so that Dad and I can work out a time for him to spend with you. It isn't the kids job to figure out the schedule. You get to focus on being a kid, I'll handle the schedule stuff with Dad."
You can also consider setting up some counseling for your kid, so that there's a neutral space for your child's feelings to be focused on and supported. We do the best we can as parents and stepparents at being empathetic and validating, yet at some level we aren't totally neutral when it comes to the other parent. So, it's not a bad thing for the kids to have extra support that isn't Mom, or Dad, or a stepparent -- someone outside the family system. Worth a thought.
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my child is involved and feeling all the feelings and it’s overwhelming.
That's a hard situation and can be a real balancing act -- how to stay in that neutral coaching role for your kid, versus "taking their side" for/against Dad, or telegraphing "your feelings are too much", or X, or Y, or Z... (not that you're doing any of that! just commenting on how it's a challenging place to be in as a parent/stepparent).
How do you typically respond when your kiddo comes to you and starts talking about feelings about Dad?