Conclusion
Kevin was showing his independence in the only way he knew how. It was really the luck of the draw that got him involved with oral sex, since it was his first experience.
But the aspect of oral sex also appealed to him once he was introduced to it, since it was as far from what his parents were trying to teach him as he knew how to get.
Even though the boy had shocked them, and he was in a bit of trouble with the law, he had finally succeeded in making them see him as an individual, and not a carbon copy of his own father.