Unfortunately there is nothing you can do, this is all about him. If he’s in denial nothing you do will help him until he’s ready to admit he’s struggling and using again and then all you can do is support him. It’s a very lonely and stressful road for the loved ones.
I wish I could give some better advise but from experience I don’t feel there’s anything you can do apart from confront with what you know and tell him honestly what that means for you and your son.