Dewi Crimp, aged 20, was arrested after his victim complained that he “scared him” and had been “poking him”.
In a text message sent after the incident, Crimp said: “I know it was wrong, I shouldn’t have done it, I’ll never do it again.”
The message added: “No need to go to the police.”
Lincoln Crown Court heard the victim, who cannot be named, was video interviewed by specially trained police officers.
In his victim impact statement, the boy said he now felt “constantly afraid” and struggled to go to sleep.
The boy’s mother said she was “sad and angry” by what had happened.
During his police interview, Crimp admitted touching his victim for no more than five seconds “out of curiosity”.
Mark Watson, mitigating, said Crimp had no previous convictions and had pleaded guilty at the first opportunity.
Mr Watson added: “This was an isolated incident in terms of his character, it was short lived and limited to touching.”
The court heard Crimp worked as stage crew in a theatre.
Crimp, of Buchanan Road, Hemswell Cliff, admitted a charge of sexual assault on April 7 this year.
He was sentenced to 12 months youth detention suspended for two years, 20 days rehabilitation and an electronically monitored curfew.
He must also register as a sex offender for ten years.
Passing sentence, Judge John Pini QC told Crimp he had escaped custody by the “skin of his teeth”.
The judge told him: “Conduct such as this on a young boy can cause a lot of harm.”
The judge also made a restraining order which prevents Crimp from going to the street where his victim lives for five years.