Ditto OnePerfectOne. My youngest brother is also bi-polar and has a drug addiction. He actually prefers to live on the streets instead of a real home, but just went into one as he was threatened with being committed to a mental facility if he didn't comply. He'll leave, he always does.
I speak from so much experience. If I or my siblings give my brother food, he'll trade it for drugs or alcohol. If we buy him a gift card to a dollar type store where he can purchase food and necessities he "loses" it, we suspect he trades it or sells it for a lower price just to get some cash. If we buy him nice jackets, boots, clothing, warm socks, sleeping bags, backpacks, etc., he "loses" them, too. Oh, and he's gotten mugged, beaten and robbed on the streets, but will run right back to them and his "friends."
If I buy him food I've learned to have him eat it in front of me, like on the way home from our Mom's funeral last November. He was literally starving, we'd found him sleeping on a bus bench just in time to say goodbye to our Mom a few days earlier. He promised her he'd get his life together, do what he had to do, and he hasn't. I and my sister make sure he eats when he's with us, we used to give him gift cards for supermarkets, fast food restaurants or dollar stores, but no more. We love him, and know we'll get "the call" one day, that our baby brother is gone.
Your brother (step or otherwise) more than likely qualifies for SSI, being that he's about to age out of his current situation I'm guessing this has been explained to him, help to complete the forms probably offered. I'm sure all the help available: nightly shelters, rehab (which is useless unless he gets into a specialized dual-diagnosis one), a list of churches that give out vouchers or gift cards to nearby stores and food, food banks, free medical care clinics, programs and opportunities available to him, etc. have been given to him. They're experts, they deal with this every day. But he has to seek the help on his own.
The bottom line is, only HE can help himself, no one else can. Gestures of blessing bags with some food and toiletries is nice, but he has to help himself with the big stuff. You can lead a horse to water...
Hugs, I know this is hard and probably not what you wanted to hear.