Hello, I can understand your concern for the sores in the mouth of your son along with blackish tongue. After analyzing the query I can get that you are quite stressed regarding the possibility of drug abuse and especially to heroine. There can be multiple possible causes including simple aphthos ulcers as well due to any internal complaints for your son’s kind of condition. In my patients with your son’s kind of situation I generally prescribe triamcinolone acetonide gel or cream to be applied over the affected area to get best relief. It is also quite important to take oral vitamin B complex for earliest improvement. You can take further guidance form your doctor. All the best.