How would the fantasy world look like where I help him individuate in his head? He starts his own relationship and mother let's him go and wishes him best of luck?
Some of it might be "let him go" in a physical sense, but much of it is internal, in his head. How he sees himself in relation to his mother, whether he can recognize that he has a life separate from her, not tied to how she feels or what she needs or wants.
A healthy mother would take steps to champion her young child to separate safely and become a whole, full distinct self independent of her. This is hard for a BPD parent. Typically, this healthy separation happens during the developmental ages between 0-6.
Having no real sense of self, and having no real boundaries, a BPD mother sees her children as an extension of her, which reverses the natural parenting order. Instead of providing emotional support to her children, she expects them to provide her with emotional support, and this stunts the individuation process for children who instinctively recognize it's unsafe to be themselves.
An enmeshed adult child like your BF may want to individuate but can't. The intensity of the guilt may be too much to tolerate -- it could feel like annihilation to try and go it alone.
In your case, it's possible that your BF, who may unconsciously fantasize to separate from his mom, transfers the responsibility for this job to you. He has a lifetime not finding the strength to do so and may see you as someone who has the x factor he needs.
It's probably not conscious.
my suspicion has been that he found me because he felt like he couldn't do it alone anymore and needed support. Is this what you mean?
He probably wants the support from a partner in dealing with her, and there are healthy versions of that. But it can also be unhealthy and there's a bunch of ways this can manifest.
For example, my H was overwhelmed by the needs of his BPD adult daughter. Instead of asserting boundaries directly with her, he would use me to take breaks from her. It took years for me to realize that he (unconsciously?) was unable to assert boundaries directly with her so he used alternative ways to create the distance he wanted. He pawned her off on me and I became an unwitting babysitter to a grown adult woman, until asserting my own boundaries.
Another example more likely in your situation is that you set boundaries about how you will engage with MIL, which on one level he admires and knows is healthy, and may even emulate for a time, but when the emotional demands of his mother increases beyond what he is capable of tolerating he blames you for things ending badly.
You showed him the path he wants to walk but you won't walk it for him and that causes him to act out toward the safer, not ballistic person, which is you.
Codependence and enmeshment run on a continuum like everything else. I don't know why some people work their way toward the light and others don't, but I do believe everyone is at least aware of the light. If the script in your BF's head is, "I see the light but every time I go toward it I get burned," it will be harder than if his script is, "I see the light and keep getting knocked off course but I have to keep trying."
He's going to feel scared. It's scary having a BPD family member. It's intense and requires tremendous strength to overcome the hard wiring. Sometimes, the best thing you can do is look for tiny little changes that things are moving in the right direction.
Edited to add: A really helpful book to read is In Search of the Real Self by James Masterson. He's a personality theorist who compares what healthy development looks like compared to the damage incurred when a primary caregiver fails to support that healthy development.
