The sexual urge in teenage boys is a VERY powerful thing. I am not making excuses for this boy, but I do take that into consideration when thinking about what might have happened. Many of our most terrible crimes are committed to fulfill an overwhelming sexual desire. Sad but true. You just can't underestimate how all-encompassing that drive can be....mixed with immaturity, poor impulse control, opportunity, curiosity...DISASTER.
Again - not to be too graphic or offensive - my thoughts are that the teenager was overcome with this desire. It doesn't take the average teenage boy very long to complete a sexual act. I don't think it would take full penetration the way you and I would think of full penetration for the act to get completed, but because the object of he's using is so little, any penetration is very harmful.