Kellogs69 can I just suggest, and I am sure you will do this, that you think about all aspects of this and discuss with your brother (if appropriate*. EG what happens if your brother adopts and then you end up not seeing much of him and much of the baby/child. How will you feel?
What if they choose to parent very differently to them? How will you feel?
What does your son know, will he know about the arrangement?
What is your main concern here? Is it finances, because I have no idea what your fiances are or what help you may be entitled to.
Are you wanting to help your brother have a family? Because, of course, much as it is a kind thing to do, it is not your responsibility to make things better for him.
Lastly, have you had any time around children or adults with Downs?
I think it would be helpful for you to do something like visit a club or school or group to find out more. You do not need to tell them exactly what your interest is (e.g. your plans) simply that you are wanting to find out more.
Just ideas, to help you think through what must be a hugely challenging time.
I am so glad your little boy is in remission. 