ESA and lack of medical certificates

Client has   a gap,  of several months, in his medical certificates which GP will not cover.  He has a current sick note but did not renew a previous one due to mental health problems.

Any ideas on how to get round this problems - he did not leave house as he was feeling too depressed - he is currently being paid ESA pending an appeal,

Thanks in advance,

Toby

Is there anyone else medically qualified who could provide him with with a backdated sicknote? EG another GP at the practice, Psychiatrist, mental health nurse