Welcome. You are not alone. I'm glad you found this sight. We all have different stories yet they are all similar so you'll find lots of support here.
Good for him for going to rehab. As his best friend, the most helpful thing that you can do is back off and let him recover - on his own. I know it's hard. But it's really the only way. He has to do this on his own right now. And you have no control over what he does. Accepting that is hard. It sucks. But it's reality.
All I can say is "this too shall pass" and life goes on. I can't predict the future but I am sure you will hear from him in time. He's not just going to disappear from your life. And if he does then remember "if you love something set it free, if it comes back to you then it was meant to be, if it doesn't then it wasn't really real in the first place."
You didn't cause his problems. You can't control his choices. You can't cure him. But you can fix YOU. We all need our own recovery. You need to recover from the unhealthy codependent aspects of your relationship and recover from your brother's death (I'm sorry by the way.)
Have you thought of al-anon for yourself? Al-anon is a program geared towards friends and family members of addicts and alcoholics. Have you read co-dependent no more by melanie beattie? This is a GREAT book. Doing these things will help prepare you for your future.
So many times we think it is the addict who needs to do the work to get better, but we do too. It is at least, if not more important that you focus on yourself right now. If youre not willing to do the work you need to do on yourself, then why should you expect it from anyone else? Examining yourself and accepting responsibility for your own mistakes is huge. Doing what it takes so you don't make the same mistakes again is even more important.
Welcome to the site! Many more posters will be along soon. But I strongly recommend al-anon and learning all you can about codependency and enabling through reading.