It is important that when you approach him you clearly let him know the dangerous effects his behavior is having on his body. You want to let him know that he can die from huffing; and do irreversible damage to major organs and his nervous system.
You may want to educate yourself on the issue first to answer any questions he may have. It is important that you seek professional advice immediately. Call a family physician, the poison control center, a school nurse, guidance counselor, or his parent(s) to discuss specific concerns and how to best treat you. You could contact the Poison Control Centers at 1-800-222-1222; or call the '1-800' number on the label of the product he is using to ask their suggestions.
In addition, the Substance Abuse and Mental Health Services Administration's Center for Substance Abuse Treatment (CSAT) [http://www.samhsa.gov/centers/csat2002/csat_frame.html] has a National Drug and Alcohol Treatment Service that provides free and confidential information on hotlines and counseling services. CSAT's toll-free phone number is 1-800-662-4357; treatment centers by state can also be located at http://www.findtreatment.samhsa.gov