Hi, I'd like to help you with your question.
It is very difficult to stand by and see your son use drugs to the point of hurting himself. No parents wants to see their children be hurt. And your son is refusing to see his use as a problem, which is called denial. It is the hardest part of drug use to break through. It does not help that your husband is not backing you up. If your son perceives his father as not against his use, then he may see it the same as approval. However, if your son can break through his denial, he has much hope for recovery. This usually happens either one of two ways, your son will "hit bottom" which means everything in his life gets bad enough that he begins to see his drug use caused it, or he gains insight from something that happens or something he sees that helps him break through.
There can be many reasons kids and young adults use drugs. It could be that your son is self medicating (he feels overwhelmed by school or other problems), peer pressure, to feel they "belong", being bored with their lives, or addiction.
Here are some things you can do to help your son:
Talk to him- let him know you care about him, love him, and want the best for him.
Tell him about how you feel- let him know you feel sad about his use and that you have noticed his grades. Don't try to make him feel guilty, but instead tell him in a matter or fact way. If he gets the feeling that you treat him like an adult, he may respond to what you are saying better.
Talk to those around him- if he is in high school, talk to his counselor. See if there are any programs to help him.
Have him see his doctor- sometimes kids or young adults will listen to those who they feel have more authority than their parents.
Make an appointment for therapy- if he will not go, go yourself, hopefully with your husband. The both of you could use support with this problem and your husband needs to hear from a professional how damaging this is to your family and especially to your son.
When you see the therapist, talk to them about an intervention. An intervention can break through your son's denial about his use to get him to see the damage he is doing to himself and his life. It involves having those involved with your son (family, friends etc) get together and confront him with how they feel about his use. Having a therapist or intervention specialist be there keeps your son from feeling defensive and helps keep the session therapeutic and helpful instead of counterproductive.
Here are some other resources to help:
Choices and Consequences: What to Do When a Teenager Uses Alcohol/Drugs by Dick Schaefer
Tough Love (Revised Edition): How Parents Can Deal with Drug Abuse by Pauline Neff
Hit By A Ton of Bricks: You're Not Alone When Your Child's on Drugs by Dr. John Vawter
You can find these books on Amazon.com or your local library may have them for you.
I hope this has helped,