Edward Graham, 64, admitted attacking the boy, 14, twice in Llandudno, Conwy.
Caernarfon Crown Court heard the assaults were historical but Graham, from Edgware, Middlesex, groomed the boy and abused him from the age of 10.
He was sentenced to three years to run alongside his current term.
An ex-scoutmaster and RAF intelligence officer serving 13 years for sexually abusing a dozen victims has received a further jail term for indecently assaulting a boy on camping trip.