The only help that you can give your cousin is to remind him that his cannabis habit is not worth getting a criminal record over.
Nor ruining his relationship with most of his closest relatives.
Cannabis is a class a drug in the uk, the maximum penalty for possession is five years imprisonment and a magistrate's court can also impose a fine up to £2,500.
If he chooses to continue buying, possessing and consuming cannabis- that shows you very clearly what his priority in life is.
If your family do decide to offer your cousin a sofa or the spare room, make sure that he does not bring any illegal substances or offensive weapons into your home.
Nor invite drug couriers or dealers around.
It isn't a crime for your cousin to live with relatives who are not his parents aged 17.
Nor for your family to take him in without his parents consent to avoid the risk of him ending up sleeping rough.
However- it is a crime for him to possess illegal substances, supply them to others and drive under the influence of illegal drugs.
It is also a crime for an occupier, managing agent or landlord to allow their premises to be used for drug taking.