Hi Roho,
I wanted to join lbj in welcoming you to the site
Your nephew is lucky to have you

It sounds like you care about him, and also want to establish some boundaries as he increases the frequency of his calls. Are you finding it hard to end the calls?
What a sad story about his dog :'(
It's a good sign that he knows he has BPD and is willing to share that with you. Unresolved loss can be processed when someone validates how we feel, and it sounds like he has not been able to do this for the past decades of his life (with the sad story of his dog being hit as an example).
Validation is a powerful communication skill that can be easy to apply, though I find myself in practice mode constantly, always trying to hit pause and validate instead of rushing into things to fix and fix and fix. The validating questions link that lbj shared was a game changer for me with my son. I had to learn to let go of two urges: one was to solve everything for him, and the other was to feel frustrated that he had no problem-solving skills. I didn't realize how much I was actually contributing to his learned helplessness by depriving him of the confidence I had in his ability to problem solve.
Let us know if you tried validating your nephew and whether you noticed any changes.
LnL