I have 2 daughters with the diagnosis.
As with many mental disorders, symptoms are similar.........so it takes some experience and "tell tale" symptoms to make a diagnosis stand out. I'd say if all docs so far are leaning toward bipolar, then that is likely......given you feel it is a "fit" as well. (you know your child better than any doctor)
The big tell tale behavior with a border line is the let me push you away so you can prove how much you love me/ I mean to you behavior. It's not fun to deal with and until you get a firm understanding of the diagnosis, it can get old mighty mighty fast......which.....actually happens at times anyway. lol
I'd read up on both as far as symptoms/behaviors go and see what you think. But most likely if he's missing the behavior above, he doesn't have it. (I will say some teens do this as typical teen behavior on a much more mild scale so be careful)
Katie: Drama queen. You're expected to come to her rescue. Hers is a more subtle version, at least with me. The moment you don't live up to her expectations or call her on behavior she is "done" with you........until she's not. She also has other issues playing into diagnosis so it complicates it more.
Nichole: Rage......and lots of it, moodiness, rapid mood swings (very rapid, like seconds at times). Nichole would say whatever popped into her head, hurtful or not......she could be vicious in a rage. Yet the moment the mood shifted all was right with the world again, and you were expected to be ok with it too. Same for behavior. If she became violent during a rage.......all should be forgotten once it was over. Like flipping a switch. She had the hardest time learning that other people did not flip moods like that and that her words behavior have a lasting effect on those around her. Took her a very long time but she did learn. She worked very hard to get better. As an adult I only see the behavior during times of extreme stress, and now I see her working hard to control it even then. She thought no one could love her........literally believed that.....for quite some time. She also truly feared abandonment.....which is why she tried to push those away she loved. If she pushed you away first in anger then maybe it wouldn't hurt so much when you were gone. Know what I mean?? Illogical thinking, skewed reality.......there version, what they perceive doesn't match what really is going on much of the time. There are still times now when she will ask me if she is perceiving something correctly......but at least she's asking.
For a while Nichole's docs thought she had bipolar too. There was one major flaw with their diagnosis, no mania, not even hypo mania. Her very first therapist pinned it on the nose, borderline personality. To see her today, you'd never believe it in a million years.
Katie? I don't think she'll ever get to the "I have a problem" part to get anywhere. ugh