From the lorax

What makes you think that the Once-ler's ideas about the use of natural resources and the manufacture of products changed?

He had once said that his mass production was becoming a huge mistake

once he began to admit his great success was in reality a mistake

Explanation:

after he began to explain what he did and saw what destruction he caused its seen that he places the "antidote" to the problem onto a boy so that he may be able to fix his error