It appears that your son has a major mental illness. In this case he has olfactory (smell) hallucinations. It should be cleared medically because such hallucinations can be a sign of serious brain illness. In most likelihood this problem is a symptom of having a mentally ill psychosis. He should be under the supervision of a psychiatrist for possible medication and psychotherapy to help deal with stressors and reduce symptoms.
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Not knowing how he reacts. it is probably safer to carry forward when hen he meets a therapist.
Thank you for caring. He is a lucky individual.