As Bednobs indicates, the landlord was obliged to place your grandson's deposit in an approved scheme. He was also obliged to inform your grandson (within 30 days from receiving the deposit from him) of the name and contact details of that scheme, together with information about the dispute resolution service they have.
If the landlord DID place the deposit into an approved scheme, your grandson should contact that scheme in order to use their dispute resolution service. As there are only three such schemes, it will be one of these links that he needs:
Deposit Protection Service:
https://www.depositprotection.com/im-a-tenant/i-want-to-know-more-about-disputes/
MyDeposits:
https://www.mydeposits.co.uk/tenants/raise-a-dispute/
Tenancy Deposit Scheme:
https://www.tenancydepositscheme.com/is-my-deposit-protected/
(Once the relevant information has been submitted on that page, there should be a link to the dispute resolution service)
If the landlord DIDN'T place the deposit into an approved scheme, your grandson should apply to the County Court to get his money back, ansd possibly more. (The court can order a landlord to pay a tenant up to three time his deposit if that deposit wasn't placed into a relevant scheme).
General information:
https://www.gov.uk/deposit-protection-schemes-and-landlords