Spell out to him how your life was back them
With respect, this is really, really bad advice.
He doesn't need to know how the OP's life was back then, he needs to know that she understands what his life was back then as the child in this situation.
When he feels heard by her; when she has 'met' him, then he will be able to hear and process it from her perspective.
we have no idea what the son is actually blaming the OP for, and I wonder if its the death of his father
Do you know what? You are actually right - we don't know why he's done it.
All I've tried to do is offer a possible insight into how he is feeling and given some context of my own experience to hopefully show I'm not completely talking out of my arse.
The advice I've given though is fairly standard.
It doesn't matter what the reason is for him cutting her off and not inviting her to the wedding, it could be something completely unrelated and nothing to do with his childhood at all.
However, the advice still stands. If she wants to move the situation forward she needs to:
A) wait for him to come round so that he is receptive to her.
B) meet him where he is. Not where she is. The OP's posts have been very OP centred - how sad she feels; people treat her like rubbish; what is wrong with her... there is little to nothing about understanding why her son has done this or concern for him. Even when she says it hurts that he can't go to her, it's still about her.
Just telling him how she feels isn't going to illicit the response she wants. If she wants to know what is going on for and with him, she needs to listen to him and she needs to be prepared to hear things she won't like without disappearing...