Hi chacha68,
Generally, telling someone they may have BPD doesn't go over well. Even when my Ex self-diagnosed with some unnamed "attachment disorder," I didn't offer my suspicion.
Your dad, however, is your relative, unlike my Ex. That's a different dynamic. It may be worth pursing why he thinks he has what he thinks he has. It can be co-morbid, and BPD usually isn't the only illness. An old family friend has BPD, c-PTSD, GAD, Depression... .; my mom has BPD, depression, PTSD (whose symptoms can be similar to BPD).
The validation tools here can work on anyone. If you can validate his feelings, he may open up. It's probably good to question what you are getting out of this though, just for your own emotional benefit. Answers for yourself, help for him, a better relationship, all of these?
This might help:
PERSPECTIVES: Telling someone that you think they have BPDIf your dad is introspective enough to think that he has "something" then that's a chink in the armor. It's hopeful, but that may all it remains. Distance certainly makes this harder, but it also sounds like it keeps you more safe as well.
Turkish