Tell him to speak to one of the major credit hire companies
If liability is clear cut ( they will advise you if they think it is - as It is not possible to asses liability properly from the brief description you have provided ) then they will provide you with a hire car , and pursue the insurers of the other party for the cost
However make sure they explain the terms and conditions of the hire to you , before signing an agreement
Having said all that , i've just noticed that he is 18 , so they may not be able to help . However if another person used his car i.e a named driver , they may provide a hire car
If in the end he cant get a hire vehicle , you are entitled to claim for loss of use from the insurers of the other party , from the accident date - if his car is not roadworthy or from the date of commencement of repairs ; if it is roadworthy , to the date of completion of repairs .