Answer

It is compared to a ‘bud’ as if it a flower or plant just starting to grow. The image is fertile and shows a great sense of life.
It then flowers ‘the bud opens’ and the power of this ‘pushes the other tongue aside’ so it takes over.

Every time he thinks he has lost his ‘mother tongue’ it comes back and ‘blossoms’ out of his mouth, showing a real sense of true identity, it is described as blossoming to show it is alive, fertile and beautiful, as blossoms on a tree are in spring.