Right. He nods at that. He knows what she's implying. He doesn't know enough about her power and that bothers him. He did a lot of research while she was gone and is an expert on fairies, if there is such a thing, but there was never much related to her power.
He follows along, picking out a pair of jeans that he deems suitable. He'll go shopping for more clothing once he's feeling himself. He looks at her seriously at her question, eyes searching hers. There's something different about her. She doesn't trust him or react to him the way she should, the way she did. She's not in love with him and he doesn't know why.
"To the best of my ability."
He's not promising to be perfect. He's not promising to be an angel, but he is promising to behave as well as he's able.