This.
He's panicking now, messaging constantly and trying every side of you to get back in. He's having a hissy fit because you've stood up for yourself, he can no longer walk over you no matter what he says and now has to adult for a change. But that's his issue now instead of yours. He's old enough, has his own property and the kids have a second home with their mother. He also has a well paid job. They're hardly out on the streets. He just doesn't want to because he's lazy and selfish. He's unlikely to meet somebody else willing to do the same and he may know this.
What a shame he didn't think and couldn't be bothered to make this much effort when you so kindly put him and his kids up for years, isn't it? You didn't want much in return, and not all if many would have tolerated that. At least not for years. You told him this again and again while he was selfish and ignored you while using you all the way around. Because you cared and wanted to work things out, but it was a one-way street that got too much in the end and you were left with no other option. He just thought you'd continue to put up with it as it's gone on for years. There were so many opportunities. All he had to do was sit down and have a proper conversation where things were sorted and contribute a bit.
YOU ARE TOO GOOD TO BE TREATED LIKE PART OF THE FURNITURE.
It's very hard at the moment. When the initial upset is over, you will get used to a calmer life and hopefully find your anger. He brought nothing to the table except misery and hard work. How dare somebody treat a kind lady like that and push your son out. It was all so unnecessary and now it's too late, but easier to blame you rather than face the consequences of his own actions. He underestimated you and you deserve better.
I agree about them not coming to the party to cause further upset and manipulation. It's not fair on your son. Good idea to sort the admin and bag their stuff up now giving an ultimatum to collect, a month maximum or it'll be dumped outside. No further discussions or answering messages. It'll help keeping you busy, get rid of reminders of them and after that you can block once and for all.