He did self-enquiry spontaneously and realized the I AM. Same situation with Ramana Maharshi who had a spontaneous awakening through self-enquiry at the age of 16.

Also see John Tan (Thusness)'s account, who awakened to I AM at the age of 17: http://awakeningtoreality.blogspot.com/2007/03/thusnesss-six-stages-of-experience.html

I had a sudden experience very similar to Eckhart's but mine happened while reading teachings on impermanence out of a Zen book only a couple days after starting to meditate. All was love, geysers of piti and sukha and a feeling of free falling through the universe. It faded within a few weeks but had caused a permanent shift and an enormous reduction in suffering.

Stumbling upon the POI maps a couple years later, it seemed fairly intuitive to diagnose it as an A&P. Eternal Now's "I AM" realization seems to tell more of the tale, however.