You don't ask for advice but for the sake of your child, I will offer you some information and encourage you to make an attempt to guide your son. Separately, I would share the following information, first with your husband and then with your son. Legally your son is an adult, but his brain is still developing. Using drugs (marijuana, booze, and other drugs) has a much more serious effect, including long-term effects, on teenagers than on adults like your poor addicted husband.

"...for teenagers, marijuana use can have much more long-term effects. The teenage brain is not yet fully mature, with neurodevelopment continuing until at least the early or mid-20s. During adolescence the brain is particularly sensitive to drug exposure, and marijuana use impacts how connections are formed within the brain. Other effects on the developing brain include interference with neurotransmitters and abnormal brain shape and structure volume.

Studies have shown that the use of marijuana is associated with reduced cognitive function in teens. One study found that teens who regularly use marijuana lose an average of 5.8 IQ points by the time they reach adulthood. A recent study found that marijuana has a more negative impact on a teenager’s cognitive development than alcohol.The Effects of Marijuana on the Teenage Brain