Fiona
If he is still in the Mandatory Reconsideration phase, then he can present a Fit Note and be excused from his JSA Claimant Commitment for up to 13 weeks.
If he has appealed the Decision, then once the DWP receive notice of the appeal he can ask for his ESA to be reinstated at the Assessment Rate, this would not be a new claim, while he waits for his appeal hearing.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems