yeah good ideas, it does have to be a gradual increase in story line and sexual evolvement between a young widowed mom and a son growing up and maturing. The storyline would need the mother and son gradually feeling sexual attraction for each other, because of the need for comfort and the closeness that sexual intimacy brings. I think the mother needs to be the one to start the awareness of what is developing between them because of the circumstances because she is more mature and sexually aware. The 18th birthday thing can be the culmination of the mother's needs and the closeness she feels for her son and his coming of age as a man, plus the sons developing sexual maturity and sensing of his mother's needs and the sexual attraction he feels for her, etc, etc. ! ??Well, I did think it would be a gradual thing. Perhaps as he gains more confidence in what he can tell her to do, she too is conflicted at each step, but decides it's important to keep her promise.
As I said, it would probably start with relatively innocent choices: What to make for dinner, what to watch on TV, etc.
He might like a short dress she wears, and occasionally starts to tell her to wear clothes that show more legs.
From there, he might tell her to freeze like a mannequin sometimes, so he can look her over.
He might eventually work up the nerve to touch her a bit while she stands frozen. (i.e. running his hands up and down her legs).
Hello or goodnight kisses might gradually get longer and longer, eventually open mouthed. Eventually, tongues would get involved.
You can guess the rest.