Pogostick,
It is okay if you are not his parents in a biological way, but you could parent him.
That is up to you two. It will be hard, very hard, and you and your fiancee will have to meet on decisions and repercussions of his drinking.
If his illness can be arrested this early, there is a good chance of total recovery, but he will have make an effort himself.
Please educate everyone in your family about what is happening and get back up.
some specialists would be a great resource for you now.
Thank you, and thank you for your little brother.
Beth