I'm (21m) worried for my brother (18m) who takes drugs and sleeps with random men

Apologies if my English's poor.
I live with my parents and attend university for mechanical engineering. My brother moved out of home 7 months ago, a little before becoming of age. He and our step-dad hated each other, there were physical attacks too. They couldn't live together.
My brother moved in with his high school friend. He lived there for only a few weeks. Then he started going to a homeless shelter, but soon he met his boyfriend (19) online. Now they're living together. The dad of brother's boyfriend feels bad for abandoning his family/cheating on the mom, so he lets them do anything.
They use the boyfriend's allowance for buying drugs, cocaine I think. My brother never did drugs before this. They also have sex with older guys from dating apps who're usually married (brother says his boyfriend has a fetish for that... don't even ask). One of them was an alcoholic with brain damage from addiction. He used to stalk them until the boyfriend's dad chased him away.
I'm sure my brother doesn't tell me half of everything that happens. He used to text me regularly after leaving because he felt lonely. Now we don't talk a lot, especially if I say critical things. The last time I saw him was two weeks ago, me and mom went to buy him new shoes. He looked really mentally down.
What can I actually do to help? How to support him? I'm afraid worse stuff will happen from now on.
Tldr: Brother is in a bad state since he has left home. Don't know how to help.


Source.