I'm sorry but there is only one option for drug dealers.

Go to the police. You can do it anonymously through Crimestoppers, or at a local station where they will do their best to keep him anonymous... but it has to be done. If the Letting Agency don't do it, then you or your son should. If there are neighbours who have complained to official bodies (police, local Environmental officer) then there will be a record somewhere which will help. Do it sooner, rather than later before they get a chance to flee.

The house next door to us was let privately, the tenants then sublet whilst they were away, to what turned out to be drug dealers. One of the girls living there was turning tricks to earn drug money - in a residential street not 500 yards from two schools. They did a moonlight flit less than 12 hours before the police raided the house. It was not pretty.

It's always hard to be the whistleblower but it must be done for the safety of your son, the neighbours and possibly fs and dd themselves.

Go to the police, tell them all you've put here and break this circle.

LTS