Your DP needs to understand that his mother isn't 'pressuring him to be the perfect son' - her demands aren't about anything he, as a son, is doing - they're about what SHE wants him to do.
The notion of "a perfect son would do things my way always" is something SHE has made up as a way to manipulate, control, and bully him.
He therefore needs to be very clear that he isn't failing in any way as a son, she is drastically failing to accept a proper mother/son relationship.
My former MIL was like this. The ONLY thing that worked was putting her on a very long time out, after telling her why, then once contact was resumed, any sign of the former behaviours put her back on the "naughty step". After a year of absolute boundary enforcement she got with the programme (mostly) and things were very much better. It took that long for her to learn that tantrumming, manipulation and bullying would no longer work. We literally treated her like we were teaching a child boundaries.
Your DP may need some support in learning what a normal, healthy relationship looks like, he may need a lot of reassurance that HE isn't the problem. This level of emotional abuse is deeply entrenched and hard to get over. But he can. He just needs to ignore her, and her voice that's in his head, telling him HE is the issue, and retrain himself to know the problem is her.
I've walked this path - he's going to need you as his biggest ally, his voice of reason, and his staunchest supporter to reaffirm that it's not him.
It sounds like you're the one to take this, and her, on.
Good luck.