Tbh I read your OP and the statement that his kids said they wished you were their mum rang instant alarm bells for me.
I wonder whether he has become too involved too quickly and that this is starting to take its tole on his kids because they’re not as settled as he wishes they were in his head.
It’s possible he was infatuated and talked up the relationship even to them to the point of you all moving in together and so on.
My ex did this to an extent. When he and his dp got together he started talking about booking “family holidays” etc and they’d only been together for a matter of weeks, the kids hadn’t even met the step parents by that point let alone each other, then when they did it was all happy families and by the time DS felt able to tell his dad that he didn’t particularly warm to the partner it came as a huge shock to ex.
There were other factors at play but ex and his dp moved in together regardless, and the difference is that he told his dp everything that DS had said to him. The upshot is that DS no longer stays there and hasn’t for several years now, and hasn’t spoken to the partner for two years.
As I say there are additional factors at play but the quickness that things moved and the need to sell it to ds were definitely a factor in the beginning.
It sounds as if your BF may in fact be having second thoughts because the kids aren’t as on board as he hoped they would be. And truth is that step parenting relationships change as time moves on anyway but the beginning is crucial.
I would walk away in this instance. I would simply call him (don’t do face to face as he becomes too emotional, but text is too impersonal) and then I would just say to him that you realise that the relationship has run its course and it’s time to move on.
Then cut contact with him. Block if you have to, but make it clear that this is it. The less contact you then have the quicker you will be able to move forward.
Good luck.