I've typically spent a great deal of time trying to reason with him, explain over and over, plead, etc. It hasn't worked of course. I think it actually adds fuel to the fire.
Yep. That's JADEing (Justify Argue Defend Explain) and, as you've discovered, it just makes things worse. I was making that same mistake before I came here. It seems logical -- "If I just explain, he'll get it and move on" but it really doesn't work that way.
It's pretty common for people to be resistant to therapy. In general, people don't like to admit there might be something wrong with them or a problem that's theirs to fix. Add BPD with its strong sense of shame and it's even more difficult. Truth is, he might never come around and agree to counseling. Or he might surprise you.
I'm glad you're feeling stronger emotionally. Keep up self-care. Work on yourself even if he won't work on himself. Plan for any contingency. Keep calm. You can weather the storm.
When you approach the subject of counseling, what do you usually say? Do you tend to bring it up in calm times or when he's dysregulating/splitting?