Albert Victor Marvin committed the offences while working as a farmhand at Boreatton Park, near Baschurch, between 1969 and 1971.
The 81-year-old, who has been at HMP Oakwood since 2012 for similar offences at another institution in Yorkshire, appeared before Shrewsbury Crown Court yesterday for sentencing after pleading guilty on October 28 this year to three child sex offences.
The court heard how Marvin worked on the secluded farm at Boreatton Park, where he would offer to buy tobacco for a teenage boy before committing sexual acts.
Robert Price, prosecuting, said: "The complainant was video interviewed in 2019. In that interview he disclosed that when he was a child, Mr Marvin sexually abused him.
"HMRC reveals Mr Marvin worked there between 1969 and 1971. Thereafter he worked at a Catholic boys' nursing home, St William's College in Yorkshire.
"At both establishments Mr Marvin was employed as an onsite farmer where he oversaw and trained boys in farming practices.
"The farm was in a remote location about a quarter of a mile away from the main building.
"The victim was one of the youngest boys there. Mr Marvin would curry favour by occasionally lending him money for tobacco and cigarettes.
"The victim worked in the piggery, a dark, secluded place, and there most of the sexual abuse took place."
The court heard Marvin moved from Boreatton Park to a similar school in Yorkshire in 1971, where he committed similar offences two years later.
Anthony Bell, defending, said: "Since lockdown in March, Mr Marvin has had no visits from any family members, although he is able to speak to them occasionally via telephone.
"The circumstances of his incarceration are that he remains in his cell for 23 hours a day."
Sentencing, Judge Anthony Lowe, said: "This was not simply committed in terms of an abuse of trust, it was in relation to a child who was already troubled and damaged.
"You must have known at the very least you were dealing with someone troubled by the fact they were in an approved school."
Marvin was given a two-year consecutive sentence onto his current sentence, to serve half.