I'm fairly familiar with bipolar disorder. I grew up with a BP cousin who had wild swings to the point if he was way manic, he would not sleep and became delusional. One of my best friends now is also BP. I would love to give you the benefit of my experience, but it's critical to know if your boyfriend is on any medications for his condition. Let us know so we can help you out.
Best of luck and get back to us with the meds info.
I hope your reception is warm but please be prepared to deal with his rejection as he is depressed right now and he may not want anyone to see him in his current state. I think you are right, just go and be with him. Don't expect this to be a magical cure for him. Wiill he throw you out? He may if he is Bipolar. Please have a back up plan if you are asked to leave his house. Maybe stay at a hotel but let him know you have not deserted him.
I wish you the best and hope your trip turns out ok.............(((((((((((HUGS)))))))))))