
I'm so sorry mama. My brother died almost 3 years ago at the age of 25. My son was only 4 months old at the time though. We didn't tell DS that he was dead, as he grew up he just knew him from pictures like he did other relatives he didn't see much. When DS had just turned 3, he asked where his Uncle lived. I told him that he died when DS was a baby (previously I'd explained death in bugs and plants, but not people), because he got very very sick and his body stopped working. DS asked a few questions, including if I was going to die, and I said yes, but not for a very long time. I could tell he was really thinking about everything carefully, but he wasn't upset. Later he told DH that he would miss him when he died, but again he wasn't upset.
Obviously it will be different for your son, since he knows your brother well. I guess I would just tell him the basics, and take your cues from him. If he asks a question you're not sure how to answer, ask him what he thinks the answer is, so you can get an idea of what he's looking for. I don't see how you could really avoid explaining death to him. I'm so sorry you have to have this discussion with your son.
I know this sounds strange, but I actually envy the fact that you know your brother is dying. My brother fell ill without warning, and was put into a medically induced coma right away. He died 2 weeks later without ever waking up. I did get to see him, but I never got a chance to talk to him and say goodbye. Make sure you tell your brother you love him, and apologize for any wrong you've ever done him.
I don't think it's necessary for you to accept that your brother is dying. Just do what you would do if you knew he was dying, but I think it's okay if you don't actually believe it. I hope that makes sense.
I'm so sorry you and your family have to go through this. Feel free to PM me if you ever need anyone to talk to. I don't know how much help I'd be, but I could at least be there to listen.