You need to read the tenancy agreement closely.It may be that your son has signed a clause which puts "Joint Responsibility" on the tenants. In circumstances like this it is common that the remaining students in the house will have to make up the shortfall.
From my own experience I found that when my kids were at uni if someone left during the year they found a replacement but the departing student had to pay the rent until the replacement was in residence.
Check the contract to see what his liability is !!