It started with Williamson touching the 12-year-old's leg, stroking his arm, before escalating to kissing and full intercourse.
When interviewed by police, the victim told them the offending took place as often as a couple of times a week.
Judge Jonathan Down called it a "peculiar case", primarily because the victim consented to the sexual activity that took place and continued to go back to Williamson's property.
"But the reality is that consent was obtained by a process of subtle development and persuasion," he said.
"It might be referred to as grooming but in the strict legal sense it was not."
Initially, Williamson denied the 10 sexual charges against him, taking the matter to judge-alone trial.
Judge Down found him guilty of seven of those charges and recently, the defendant had accepted responsibility for his actions.
A year was deducted from his sentence for his newfound remorse and two more years were taken away to reflect the payment and Williamson's previous good character, attested to in affidavits from various friends and family.
But Judge Down was quick to point out the psychological damage to the victim, which was exacerbated by having to give evidence at trial.
"He slept a lot and missed out on school and other activities... I expect it interrupted his education to a very large extent," he said.
"This type of offending is far too common. [The boy] will struggle with this for many years to come and it will have had a significant effect on the way he views himself."
Judge Down refused to impose a minimum period of imprisonment.
