I believe that the episode with your niece has little to do with your son's drug addiction now. Your niece is not demanding cunnilingus now, and your son is a grown up man by now.
If you son wants to get off drugs he will need to treat the underlying biological abnormality that cause him to be addicted to drugs for relief of his symptoms.
Most drug addicts are hypoglycemic. This causes the over-production of stress hormones for which your son is taking calming chemicals (drugs) to counteract the feelings of anxiety and discomfort. Most drug addicts are hypoglycemic.
This illness an be treated without recourse to drugs by going on a Hypoglycemic Diet.
Also read:
Drug Addiction is a Nutritional Disorder