I believe the "prosecutorial discretion" to prefer a lesser charge against him was exercised after some intense [plea] bargaining before the charges were even brought to light.
In the end, he chose to plead guilty [to a lesser charge] in order to avoid rotan on his butt cheeks..
"The difference between them, however, is that Section 376A(2)(a) applies to an offender who was in a relationship exploitative of the victim. If sentenced under this provision, an offender can be jailed for up to 20 years and fined or caned."
Ian was the victim's mentor in the acting school, and it's arguable that he was in a relationship exploitative of the victim, which would have earned him a good caning.