The only way to really understand your son is to ask him. I also think you should probably try to ask him in person or on the phone. The written word can be a lot harder to understand or to use for serious, in depth conversations. Perhaps you could suggest meeting up for lunch/dinner/coffee just to talk.

From what you said and from what your son's text says, I think there has been a misunderstanding or a miscommunication. It sounds like you did a lot to show your support for your children, but he might have needed support in a different way. I think you should ask him what exactly he wants from you. What is this "what is necessary" mean, entail? What is it in his mind? What does a parent-child relationship look like to him? Maybe to him, showing love and support for him is actually taking care of yourself and receiving treatment for your Bipolar? But again, there's really no way of knowing what is going on in his mind without asking him.

I do have to say, I don't agree that a parent-child relationship is all on the parent. I am not a parent, but I am a little bit older than your son. And I believe all relationships require give and take from both sides. Just because my mother is my mom doesn't mean I shouldn't be understanding of what is going on in her life. But at the same time, she should be understanding of what is going on in my life. We've gone through rough patches, but I believe we have always had and always will have a very strong relationship because we make a point of communicating with each other (and apologizing when we need to, both of us). And not just the events of what's happening, but talking to each other like "real" people, real adults. While being young and dependent on my mother was a very special relationship, I think being an adult, on equal footing with her is a new, exciting experience where parent and child become more than just parent and child, they become friends... I invite her to visit me not out of filial obligation, but because I enjoy spending time with her as a person that has similar interests to mine.

I'm sorry if I wasn't much help. I hope you are able to build a relationship with your son again. Just remember to take care of yourself first.