I'm really not sure how people on a website diagnosing him with 'definite severe brain damage' is in anyway helpful? If anything it will just cause further depression and possibly continued drug use because why not if you are already screwed? We don't know how he will respond.
I guarantee being positive and staying completely sober will be the best thing for him. And giving himself time and trying to use his mind for more challenging things is better than doing nothing. It's when people are negative and give up that there isn't much progress or they decline.
He is also 19 which gives him better odds of recovery than a 40 yr old.
OP, I wouldn't take any of what we say completely to heart. I was told when I was 23 that I had a disease that kills 80% of people in the first year. I could have listened and become depressed and probably added to that statistic in a self fulfilling prophecy. But I stayed positive and worked to stay healthy and give myself every opportunity. That was almost 14 years ago and I still have the disease in addition to another that is related. But I deal with them well and it seems that every year I get a little better, at least symptoms wise. But I do everything I can to make my body stronger. My point is, we never know what the future holds. And it's much better to remain positive and live healthy and do everything you can to make things better. The brain is an amazing thing and can overcome some crazy things, yet it can make a body sick when there is nothing wrong because someone is negative and believes something enough. But being positive, optimistic, and happy is a much better way to live than negative and depressed. If you stay positive, sober, and give yourself plenty of time to heal (years even), you will be giving yourself the best odds of recovery.
As for the medical costs, see if there is a large University or medical center near by. Many are doing research on your very situation. If so, you may have to be a lab rat and let them run tons of tests all the time, but they should also pay for it all to help with their research and may even compensate you for time and travel expenses. It's up to you to be proactive and try to find things that can help you. Also, talk to your Dr and ask if he knows of anything like I mentioned or if he knows anyone that might know, that he can call. You can also make phone calls and ask. It will probably take many hours of calls as you will be given the name and number of someone that might know or be able to help and so on and so on.
Also, there are some brilliant doctors that will be able to tell you what you can do to help yourself and that are optimistic. But there are plenty more that aren't all that bright and have terrible bed side manners and will be depressing to talk to. So don't take everything they say to heart either and hopefully you can find one of the good ones.
If you have a hard time remembering things, start carrying a pen and notebook with you and write stuff down you want to remember. I make lists all the time because none of us remember everything and it's an effective tool to make sure you do things you need to or have everything you will need for a trip, etc, etc.
This is my opinion based on having severe health issues and having to deal with many doctors in many places and having to see a Dr once a week or more for 14 years now. I'm sure others won't agree with it all. But you can either try and make things as good as they can be, or you can give up and not get very far. And that goes for anything in life.
Good luck and try to stay positive no matter what!