Every Buddha has to start out as a bodhisattva. A bodhisattva is basically a Buddha-to-be.
I thought a bodhisattva was one who voluntarily delayed his transition to nirvana to help other beings achieve enlightenment.
Sometimes it is presented this way, but the general Mahayana teaching is that a bodhisattva is someone who vows to become a Buddha for the enlightenment and liberation of countless sentient beings. This is in contrast to the Hinayana ideal of the arhat, who attains a personal enlightenment.
There is no need to delay Buddhahood to help other beings achieve enlightenment, because that is what Buddhas do.
Wait...Buddhas do things? Clearly everything I thought I knew about Buddhism is wrong. I need to start another thread for this.
Different Mahayana Buddhist schools have different ideas about what a Buddha can do. Some say that once you become a Buddha, you can't help sentient beings as directly as you could as a Bodhisattva, so one should focus on being a helpful Bodhisattva and let Buddhahood take care of itself.
Others say that being a Buddha means that one has the greatest power to help sentient beings, so one should realize Buddhahood as quickly as possible.