Okay, firstly if your son has paid his rent until the end of his tenancy agreement then the landlord cannot permit occupation by another until the tenancy has expired and served a valid s21 Notice for Possession under Housing Act 1988 upon your son. He has a right to quiet enjoyment of the property until, at least, the end of the fixed term.
He will be able to sue the landlord to recover the loss suffered as a result of the landlord reaching his right to quiet enjoyment by permitted workers in and, if the tenant occupies today, a new tenant in the property. If the tenant does not occupy then your son should move back in for the remainder of the term of the tenancy.
You will be able to recover at least some of the money you have spent in arranging a hotel, although the amount for two nights does seem on the high side. After your son has recovered his possessions and moved back in (if he is able) then he should write a letter before action to the landlord giving the details of the issue and evidence of the loss you have suffered (receipt) and asking that he compensate him for the full amount within 7 days of the date of the letter. If the landlord suggests compromising on the figure then would negotiate - 1) to avoid the cost/stress of litigating and 2) because a court might very the amount you paid as on the high side and aware you less.
If he does not offer to pay then you will be force to issue in the small claims court, www.moneyclaim.gov.uk
If he cannot get back in to the property then you should move him in to cheaper accommodation until the term of the tenancy has expired and write as above but in respect of the additional money you have had to pay to house him until the term expires.
At the same times as writing to him ask for details of the tenancy deposit scheme in to which his deposit has been placed - if he is occupying the property under an assured shorthold tenancy then his deposit must have been put in to a scheme, if it has not your son will be able to sue the landlord for the return of all of the deposit and a fine payable from the landlord to him of three times the deposit amount.
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