Is there any way to get him in to rehab? I feel like if he's that worried about what he might do next professional help might not be a bad idea. How much you want to help him might also depend on how many bridges he's burned in the past though.
I have some cousins who had a rough go, and two of them got good girlfriends and smartened up, the other two resent their family and parents and take no responsibility for what they do. Its tough because they become not likeable and you stop wanting to help them because they reject you.
Anyway, I hope you figure something out. I would say take him on or at least be there for him if he's really trying to clean up his act, but if he's not ready to take responsibility for what he's doing it might be harder than you expect. Try and talk your parent's into helping too if thats what you decide to do. You may make a better case than your brother. Good luck, hope you make a good call.