It was either gaining the ability to walk in the sun and losing you, or keeping you at his side. He's always been quick with making decisions, acting on what he needed to go to get what he wanted, but now he's at a standstill.

You gained the ability to walk in the sun, how it was done is unknown. He could easily devour you to gain the ability, but it comes at a cost. He loved you. He's never felt this way about anyone before, and if he wanted to conquer the sun, he'd lose you in the process.