The council have a duty to help . If I was you I would first phone the ‘assisted housing’ to clarify , then I would call the council dept for homeless for advice , then shelter.
Your son can also phone his local citizens advice for advice , they know the law inside out .
If he was in fact dealing drugs they would need proof before they evict him. If they have proof then the council may deem him to of made himself homeless and won’t help him so shelter and CAB would be his way forward.
2 weeks ago I went on anti depressants and have slept better. Xx