I think she already knows she's the only one in his world, it's just she misunderstood his "love" for her as a family love, like a baby chick towards its mom or a little boy towards his sister. She must felt like her first day dropping him off at school or something when he starts to expand his social circle and still doesn't want to leave her side. It's different from grooming. Grooming is when you implace yourself as the only one and to harvest later, this is her knowing she is the only one but she doesn't want that, she wants him to grow and she'll know her position like this won't last forever. Kind of like a parents knowing that when the child reach a certain point their words will start to be less important.
About him though, lmao he just loves her as a woman and trying to mature fast. Well trying to pass age 17 and I'll cheer you on!!