Emerging emotions can be a wonderful opportunity for a writer, especially if you plot out that it is going to happen before you actually start putting words down on paper.
In such a situation, as you craft the pages which precede the character's emotional awakening, you can restrain your natural descriptive and emotive writing style. You can make the writing factual, sterile and subtly gray. Whenever your pre-emotive character takes on the POV, your writing should reflect his limited understanding of the world, omitting all that he cannot perceive not only through the content of what you write, but also in the very style with which you write his character's view.
Then, once his emotions have been set free, unfetter your own gifts and let the colors, textures and harmonies of life pour out onto the page. Don't tell us that he can suddenly "feel happy". Show us how spectacular the world suddenly appears to his long-shaded eyes.
The best example of this kind of transition comes from film rather than literature. Go watch "The Wizard of Oz" from the beginning and pay attention to the "not-so-subtle" difference between Kansas and Oz. Then go do the same thing within your story.