I share a 4yo with my ex.
We celebrate Christmas, Easter, DS’ birthday and sometimes new year together.
Ex doesn’t really mark adult birthdays, nor do I, so we don’t get together for those- although I’m sure we would if we did IYSWIM?
There is nothing romantic between us, but we take the view that although we broke up, we are still a family- were DS’ family- , if that makes sense? And we get together for him, rather than for ourselves- though we do enjoy one another’s company.
And when ex had a girlfriend, we talked about her joining us for these occasions once exDP was sure it was serious (turned out it wasn’t serious, unfortunately).
Anyway- all I’m saying is - families - together and separated- look like all kinds of things. Some get on well, like ours, others not so well. The question really is not whether what he’s doing is right, but whether it’s right for you- and if not, you have to decide whether you stay and put up or go.
It’s tough though, OP, and you have my sympathy.