Oh AnaBanana I'm so sorry to hear about Fletch. He sounds like a most wonderful dog and family member. I really like Glitternerfball's suggestions of celebration before the day comes. All you can really do is be there for your brother when Fletch decides it's time to go "home".
My beloved Snowflake, was very ill for several years. She was my dearest baby girl. She was diagnosed with a heart illness, then went blind in one eye, then had daily injections for kidney failure, then had a partial stroke. I just couldn't let her go, even though she was becoming so ill. She would still try to get up and greet me and sleep with me every night. One night, I realized that my begging her to stay, really wasn't fair for her. And I held her tight and told her it was alright for her to go to kitty heaven. And that I would be ok. And I would always love her. I gave her permission to go. The next day she died in my arms. And I sincerely believe that she had stayed around so long to get me through some of my most difficult times.
I belive that Fletch is totally aware of his state of being. And also genuinely knows how beloved he is to your dear brother and your family. Your brother may want to say his goodbye while Fletch is still alive to hear him. Making plans ahead of time certainly helps when your heart has broken. Your brother will need his strength to let Fletch go in the physical world, even though his dear puppy will always be with him in the emotional one.
I now have plans in place for my two kitts should anything happen to me, or to them. I have their containers for their ashes that match Snowflakes and they will all be buried with me when that time comes. It's something one doesn't want to think about, because the thought of loss and grieving is so terribly heartbreaking and personal. Pets can never be replaced. Just reunited with you later on.
May Fletch leave this world in love.