the first thing i know is that when someone is having a bipolar episode they are often not too rational. i think under normal circumstances, a rational discussion of why you kept your account separate would suffice, however right now since it seems like its not working i would just try to take the focus off the issue altogether. ask him if you can help him, maybe even offer to lie down next to him (or just do it!) when he seems to be just "lying around" because sometimes i know im lying down feeling so lonely, even if my bf is in the same room, but if he would come hold me it would really mean the world. the sadness and loneliness can be overwhelming, literally to the point of not being about to get up. if you think you can make it through this with him and he is someone worth the extra effort you may make, and the ups and downs for a bit, then give him your all.

also i do know when i get fixated on certain things its very hard to get me to calm down and get off the topic. but a big big hug never hurt to try. just saying "im here for you and want to help you" i know to me, would mean a lot, so id assume someone in a similar mind frame would find that to be something they would like to hear.

bipolar people experience things very NOW NOW NOW when they are in a certain stage, so try to keep that in mind, that any thoughts of even 5  minutes in the future are probably not crossing his mind. does this help you at all? my biggest suggestion is to really give yourself to him (assuming hes worth it once youve made it through this) and ask what you can do for him or if theres anything he needs. he may be completely unresponsive, but at minimum when he comes out of this stage he may remember that you were there caring for him and it will only strengthen your bond.