Claiming IS while appealing ESA

I have a client who would be much better off claiming IS while appealing ESA decision because he gets DLA himself and CA for looking after his son.  I have had problems with the procedure of doing this before as the ESA claim has to stop before IS can be paid.

In my last case, CA also had to be changed from underlying entitlement to payment, but this chap already gets a small amount of Carer’s Allowance in addition to C-based ESA (don’t know why as overlaps with his c-based ESA).  Despite writing to CA, ESA and IS to ask them to co-ordinate benefits so that client (who has learning difficulties)  did not have a period without money, this did not happen last time and he was left without money for about 3 weeks. 

My current client is reluctant to move to IS as he simply cannot afford any delay in payments.  Overall, he would be slightly better off on IS in any case, assuming his son (who is 15) continues to get MRC DLA after his 16th birthday.  I spoke to someone at ESA who said it was difficult for them if he did not claim ESA while appealing, as they have no open claim and do not know what to do with med certs.  She suggested clients on IS/JSA should keep their med certs and send them in only if the appeal succeeds.

Does anyone know a foolproof way of dealing with this?