A concerned dad used his son's Facebook account to catch a Scout leader making sexual advances to the teenager.
The father found explicit conversations between Christopher Pimm and his 15-year-old son on the social networking site, but instead of going straight to the police, he allowed the messages to continue while he monitored them.
His surveillance eventually provided officers with enough evidence to arrest Pimm.
Prosecutor Richard Scott told Basildon Crown Court that after the arrest last year, similar approaches to two other boys, aged 13 and 14, were found.
"The father of one of the boys had been monitoring the contact on Facebook and saw the sexual nature of it," Mr Scott said.
"He became concerned and initially instructed his son to cease all contact.
"But because he had concerns about perceived predatory behaviour he allowed his son to remain in contact with Mr Pimm and later reported the nature of that contact to police."
Pimm, 24, who was also a sailing instructor, had met all three of the boys in person either before or after making the sexual approaches.
During the conversations, Pimm sent the boys links to internet pornography, questioned them about their sexuality and made lewd suggestions.
When his computer was seized, police discovered 851 pornographic images and videos of boys aged between eight and 15.
Pimm, of Great Fox Meadow, Essex, admitted three counts of causing children to watch sexual acts, two of inciting a child to engage in sexual activity, and two of possessing pornographic images of children.
Judge William Graham sentenced Pimm to serve a three year community order under which he will be supervised and required to complete a sex offenders programme.
He said: "What started as innocent chat moved on to talk about sexual matters which were completely inappropriate.
"These are serious offences, particularly because of the number of boys you approached and the persistence of your approaches."
Pimm will also be banned from using the internet without his activity being monitored, and was ordered to pay £1,000 in prosecution costs. He was placed on the sex offenders register for five years.