How do I be supportive?

I just found out this morning that my husband of 24 years is snorting pain pills, he doesn't have a prescription for them he gets them from a friend who does. He says he's been doing it for about 4 months but the problem has definitely increased over the last month. I guess there were signs I didn't pick up on at first. But now, unfortunately my eyes are wide open. He says he's glad I know, he was trying to figure out how to tell me, I don't believe that. Anyhow, my issue, I don't know how to be supportive, I'm so friggin pissed off at him and the situation I can't even see straight. He doesn't want me to say anything to anyone just keep in bottled up. I'm not so good at that though. I won't betray his trust for now but if I see things aren't getting better I will seek out others in getting him some help. Adding to my anger is so many of the people around me I've realized with this happening have drug issues of their own. Any suggestions as to how to be supportive and not let my anger show?