By saying that you are like a mother to him and that you treat him like a son, he most likely meant that you give him a lot of affection and attention.
In telling you that he has never been treated like this by any woman, he most likely meant that he has never had a woman love or care for him in the same way that you do.
None of this means that he only looks at you as a mother figure.
He does look at you as a lover.
He’s just not used to the kind of affection and caring that you show him.