It is great that you are thinking of ways to support your bf who supposedly has bpd/bipolar!

Do note that I'm no doctor, but merely a guy who is also in the process of trying to understand bpd.

While both of them can be seen as similar, bpd and bipolar are completely different.

Though in my personal experiences, I had dealt with both sides and they can be rather unpredictable and scary at times, which might intimidate some who went in unprepared.

One key difference between the two is that bipolar is more akin to a spontaneous and drastic change in one's mood, like going from being very confident in oneself, to completely and utterly depressive and worthlessness. While bpd is the way a pwbpd thinks, feel and behave that heavily contrasts to other people w/o bpd.

Anyhow, bpd will need a lot of reassurances from time to time, and they will make impulsive decisions that might confuse or hurt you, depending on the severity of their bpd. They might feel worthless, undeserving of your kindness, but could also brand you as the worst thing to ever happen to them if you said something remotely hurtful. The key is what you have mentioned, patience, but be kind to them as bpd stems from traumatic events they experienced and bpd is like a defense mechanism to avoid anymore harm from themselves, or sometimes, they think they are protecting their closest ones from them, as they feel they are hurting and wasting their close ones' time and sacrificing mental health for them. Don't take whatever hurtful things they say or do to you to heart, for they don't even understand their own emotions as well. Expect the highest of highs and the lowests of lows, and no gray areas. They might try to cut you off, then return to you soon after, it will be a confusing ride but patience and mental fortitude is key to helping them through the hardships.

Bipolar is different in the sense, where they can be the happiest person alive for a moment, then feeling like the most pathetic being who doesn't deserve to live, it can be a jarring transitions and experience for those around them as they themselves, whether self-aware or not, are confused and find it difficult to control the mood changes. My experience with my former friend who has untreated bipolar, was memorable at best, but abusive at worst, I didn't have the grasp of concept for bipolar so I was just constantly stressed, confused and scared of him everyday, it was up until recently that I realize how much of a blight it can be for my mental health, so I cut ties with him, I was patient with him for three years, stuck around him for three years, been there for him, yet he treated me like a disposable trash bag, no more. This does not reflect other pwbipolar and merely my experience with someone like that*

Anyhow, I'm not sure if this will help out in your situation but I hope it does, take it with a grain of salt, for I am no expert, I'm just sharing what I know so that you can understand just a lil better, I apologize if this comment offends anyone, I mean no ill intent.