Sounds like your son will benefit from two websites.
Look on Facebook for the "UK Landlords Support" group. They are an absolute minefield of landlord queries & how to get your property back. Secondly, he should explore joining the NRLA. It's a small membership fee for endless support & legal advice to Landlords.
At present he could serve a Section 8 for non payment of rent or a Section 21 for a repossession. If he is owed rent he can go for a Money Collection Order (MCOL) he could also, long term, get a County Court Judgement (CCJ) against the tenants which will really impact on their credit record & mess them up.
Right now evictions are taking, by law, at least six months due to C19 but if he acts fast that will fly by. The courts are seriously backed up but if he's got all the legal paperwork in place, an eviction should go through. There's a long list of legal obligations and certificates: EPC. Gas Safety Certificate up to date, How to Rent booklet, deposit with a Govt recognised deposit system, current Electrical Safety Inspection report etc. Check out the GOV.UK website for more advice. Just enter "tenants" or "tenancy law". Something like that will bring up loads of govt advice/rules/regs etc.
Good luck. Don't delay. Get the process started. Some landlords, like many others, are having a tough time right now. They have mortgages to pay but no rent coming in to pay it. I have good tenants but some (proven) unscrupulous tenants are taking advantage and screwing over their landlord.