Police raid found cannabis plants and now freeholder wants property back?

Hello everyone, My son bought a leasehold flat in North West London and the freeholder has been sending him numerous letters from his solicitor alleging the flat is not maintained etc. All untrue as my son spent £20,000 on refurbishment

The freeholder is trying to get my son to sell the flat to him. He did the same to the previous owner although we do not have his address. The flat has been rented for 3 years to Romanian electricians but my son never checks the flat as long as they pay the rent.

Two months ago the police raided the flat and found some cannabis plants and now the freeholder is trying to use this against my son to take the flat away from him.

The funny thing is he is asking my son to repair the roof whereas my son has bought another flat in Effingham where the same freeholder sold him the freehold but has a flat leasehold there and he is asking my son to pay for the roof there where the situations are reversed on this property.

There are 50 years left on the lease and the freeholder is threatening to take his flat.

Does anyone have any recommendations or legal advice that could assist?

Many thanks

Allen