I think I read the story on literotica but I am not so sure. I remember, that there was a son that his mother was a constant tease, and kept asking him if she looked sexy in specific clothes before going out on dates. I think that one time after one of her dates, she returns home drunk, teases her son once again and he forces himself to his mom,telling her that this is what she wanted, although she now tells him that it is a mistake.
Source: reddit.com/r/Erotica/comments/ehrp6u/looking_for_a_specific_momson_story
Have you tried using multiple tags to narrow it down on literotica? I’d try “forced” and “incest”
https://www.literotica.com/s/a-mother-and-her-son