My heart goes out both to you and your stepson, because he is on a dead-end road right now that can only lead to disaster. Unfortunately, he probably refuses to face it.
Your question isn't easy to answer, because every person is different and so is every situation. However, as a general rule, I urge parents to "go the second mile" with wayward children—in other words, keeping some relationship with them if possible. If we drive them away, we may never have another opportunity to help them. Young people seldom realize how evil the world can be, and can end up in terrible situations if they leave home.
Let your stepson know you love him, and that you and your husband are deeply concerned for his future. Realize also that he may not be able to stop taking drugs on his own; some drugs can be very addictive. Instead, find out from your pastor or doctor what treatment programs are available in your community, and do all you can to get him involved in one.
The most important thing you can do, however, is to pray for your stepson and urge him to turn his life over to Christ. God loves him, and Christ can give him new power and a new purpose in life. Only Christ can turn us "from darkness to light, and from the power of Satan to God"‐and this is what he needs (Acts 26:18).