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When he left the lay life, he didn't have any attainment(Nibbana) yet. Thinking to leave the lay life is called Nekkhamma Sankappa. It's a part of Samma Sankappa, the right thought of the noble eight fold path.
Even ordinary people can get Nekkhamma thoughts from time to time. But those are often not strong enough to make them give up the lay life.
The common understanding is that the Buddha attained all stages of enlightenment under the Bodhi tree on the same morning he became the Buddha.
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[unconfirmed, proceed with caution] since in his previous lives he already was a bodhisatta and so a noble disciple, becoming awakened was only a matter of time since according to the Dhamma after attainment of the sotapatti noble level (the 1st of the four) nibbana is to be attained within 7 lives at the most.
thus the Buddha was predestined to become one. even he himself couldn't change that. it's only that all his previous practice and wholesome kamma ripened in that particular life 2500 years ago
until the awakening he was not a buddha, but an unawakened bodhisatta
before my Awakening, when I was an unawakened bodhisatta
Ariyapariyesana sutta (MN 26) by Ven Thanissaro Bhikkhu