Graeme Mulock was responsible for the abuse between 1 December 2016 and 25 December 2018 in the Rochester area.
Kent Police was first informed of the allegations in 2018, and an investigation identified Mulock had targeted his young victim on three occasions.
Mulock, of Yantlet Drive, Strood was interviewed and denied the allegations. He was later charged with four sexual offences including two counts of sexual assault of a child.
The 65-year-old pleaded not guilty at Maidstone Crown Court and after a three-day trial he was found guilty by a jury, on Thursday 24 June 2021.
The following day, Mulock was sentenced to a total of three years and six months’ imprisonment. He will also be added to the sex offenders register for life.
Detective Constable Judith Edwards, of Kent Police’s Vulnerability Investigation Team, said: ‘Mulock acted despicably by targeting a vulnerable child over a period of two years. He is undoubtedly a danger to children and it is only right that he is now serving a prison sentence.
‘I would like to take this opportunity to commend his young victim who has shown considerable bravery in assisting us to bring Mulock to justice.’