CG
Starting work will affect all of his benefits potentially.
Obviously, he will need to close his ESA claim as working 18 hours a week will mean that he is considered as being in remunerative work.
His Housing benefit may be reduced due to his earnings, it may even be removed.
Whilst there is no specific requirement for him to notify the PIP department that he is starting work it may be to his disadvantage if he does not.
If his starting work is the result of an improvement in his conditions then he must notify PIP about the change, this will result in a full reassessment of his claim.
I would recommend that he puts his current information and that from the proposed working into one of the online benefit calculators to understand the impact.
www.gov.uk/benefits-calculators Gordon