What is a reasonable punsihment for cathing my child smoke weed?

Counseling . They tend to follow their peer group. Make a conference with his md so he can explain to him the bad effects of smoking weed and other drugs. That it can affect the heart and lead to addiction.
Smoking weed. This is a serious problem and you require professional help in order to try to stop your child from using drugs. Call your doctor and have your child examined and then have the doctor refer you and him for proper preventive treatment.