Sorry, none of you have it right! Which idiot flying school gave you such abysmal guidance?
Firstly, what is the expiry date of his licence, not Rating. If it was a UK pre JAR-FCL PPL it will probably be a lifetime licence. But if it's a JAR-FCL PPL, it will only have been issued for 5 years.
Assuming that the licence is still valid, then the SEP Class Rating must be renewed by a Renewal LST. No exams, just whatever training is thought necessary to pass the Renewal LST.
If the Rating was valid less than 5 years ago, the Examiner will renew the Rating immediately after the LST, there will be no fee to pay to the CAA and the only thing which needs to be sent to the CAA is the form SRG 1119. If the Rating had expired by 5 years or more, the renewal would have to be done by the CAA and there would be a Rating variation fee to pay (£70).
If the licence itself has expired, complete the LST as above but send off licence, medical, SRG 1119 and another form (SRG 1102) for licence re-issue (although some dozy Belgranist has incorrectly termed it a 'licence renewal' form), together with the licence re-issue fee (£59).