@babyzoomer if Unite contracts are the same across all accommodations then the OP will have signed as guarantor and it is a legal and binding contract. The "child" is over the age of 18 and there are consequences like this to dropping out of university. So it doesn't matter that he isn't a full time student because Unite have a clause for that.
Unite state in their contracts that it is your responsibility to find a replacement tenant to take over the room. Your contract is then ended and a new contract is set up for the new tenant. He may be lucky and they have a waiting list so they can fill the room but ultimately he is responsible for finding a replacement tenant.
Yes once he ceases to be a full time student he is in breach of his tenancy agreement and cannot occupy the room (from their terms and conditions because my own child was in a Unite room so I have all the documents) they can apply to the court to get the room back if the non-student is still in residence.
And the kicker "The Guarantor guarantees that you will perform and observe your obligations under the Tenancy Agreement (including payment of Rent). If you do not perform and observe your obligations, the Guarantor will do so instead. The Guarantor is liable to reimburse us for any unpaid rent, permitted fees you have been charged and any reasonable costs and expenses (which must
be reasonable both in amount and in nature) which we have properly incurred, where you have not carried out your obligations under this Tenancy Agreement. We are under no obligation to bring any claims against you before bringing any action against the Guarantor."
So whoever signed as guarantor is on the hook for the rent. In fact they emailed me every time a payment was due to remind me to make sure there was enough money in whatever account the rent was coming out of. They need to find a new tenant.
@notnorman Contact them and see if they have anyone on their waiting list that can fill the room. If not advertise it at the university, on any student letting site you can.