Kathmandu,
The behaviors you describe do sound like BPD, possibly "High Functioning" BPD. The "High Functioning" distinction is important, if it applies to your fiancé. If you're not sure, reread the section about High Functioning BPD in the SWoES book. This is an important step to answer your question "is it possible to help him?"
BPD people tend to resist the possibility that they may have BPD, particularly if they are High Functioning. If he is, he won't want help because nothing is wrong with him, from his perspective.
You can't change him, but you can change how you respond to his outbursts. The SWoES book has some guidelines about this, and the folks in this forum have a lot of experience dealing with the borderlines in their lives so you're in the right place.
You might want to start with this:
https://bpdfamily.com/content/ending-conflict Hope that helps, it helped me a lot.
I have also benefitted from logging in here and reading posts regularly.
DoubleBP