He wanted to follow his passion of teaching children to draw. He set a goal of teaching 1,000,000 children to draw by the age of 18. Stopping short at 400,000 on his 18th birthday re-set his goal to hit the million mark at 21 and continued teaching hundreds of kids at schools.
The passage describes someone who is passionate about teaching children to draw and has set ambitious goals, which he adjusts as he progresses.