student guarantee

Guarantor

i have been a landlord for many years  and met many scenarios but now im at the otherend. my son has signed an ast with a private stuent provider  on a residency  until august  The managing agent for the landlord is almerostudent.  after he signed the ast he was told that he had to have a guarantor if he wasnt to be asked to pay all of the rent up front. As he didnt have a guarantor arranged they refused to give him keys. Even now  after 2 months of non residence and after he has paid  over 3000£ to the managing agents they are still refusing him the keys. They have sent me an email inviting me to be a guarantor but refused to give any details of what i would be guaranteeing. I have now obtained  a copy of the rental contract from my son  but still no more info from the managing agent. 

When i have required a guarantor ive done evrything upfront and the guarantor has signed the rental contract at the same time as the tenant.  I find the position i find myself in now very disconcerting and borderlin illegal in that they have pressurixed an 18 year old into signing an ast which will be a legal bind over him for 10 months.  and have not themselves handed over any accomodation  as their part of the contract. My son assures me he hasnt signed any other document with them apart from the ast. Is the normal for student lettings? How can o approach this/?