This is actually a rare occurrence, as most games have the MC trying to conquer his mom. Mothers that initiate sexual content do seem to come up in games with shota protagonists, but I avoid those so I couldn't tell you. Apart from these, the most prominent example of a mom romantically pursuing her son is of course Linda from Milfy City that others have recommended and I do too. Everyone likes to shit on this game, but it is actually quite good. The developer sucks big-time, but all his work is high quality and was trend-setting when it first came out.
Another title that kind of fits is Kindread: The redburns. Here there is no romance though. The mom has a deal with the son (MC) to relieve his urges (via handjobs, which there are a lot of), in order to avoid bro-sis shenanigans during a zombie-apocalypse. She is very strong willed and calls all the shots in the beginning, however the dynamic is reversed over-time, with the son constantly pushing her boundaries and taking control of the situation (the sexual stuff, not the everyday interactions). The character building and the writing is phenomenal, but unfortunately the game's development stopped after reaching the BJ stage.
If not limited to games, there are similar examples in other media. A few widely different ones that I remember from the top of my head:
- Love Mommy (2016 short film): Hot mom has some mental issues and is really dependent on her son emotionally. She moves in with him, is jealous of his gf and tension rises. She provokes him into fucking her over the dinner table. Then sneaks into his bed, while he is sleeping and rides him. That moron actually left her in the end, but I guess they wanted to pass a message. Very good overall.
- Mom's helping hand (porn scene from PureTaboo studio, so it was toned down as step-family): Son breaks both his hands and his stepmother offers (by herself, out of the blue) to help him relieve some tension. Starts with handjobs, progresses to oral and finally vaginal. This setup must be a trope at this point, but the MILF is hot and the scene was filmed very well. More importantly, it is actually based on a real story posted on Reddit long ago, about a biological mom-son couple (and consenting cuck father). The story is probably hotter than any re-enacting done since, but is a pain in the ass to string together from the Q&As.
- A motherless son (Sex&Submission): Son returns home after a long time, hot MILF confesses that he is actually adopted, then she goes out drinking and, upon returning drunk, proceeds to give him oral, while he sleeps. Next day he is pissed (wtf?) and from that point on, it devolves into heavy maledom. Porn logic, while there is any, and then mindless maledom BDSM. The MILF is hot though (younger Syden De Mer).
- Yokorenbo - Immoral Mother (hentai): Widowed and sexually frustrated MILF sleep rapes her asleep son. Here at least, there is biological mom-son incest, since Japanese are not as squeemish about any kinks (they draw the line before uncensored genitalia though, go figure). Then the MILF gets remarried, but the son decides to reclaim her, not always consensually, along with his bitchy step-sister. Still porn, held back by its porn logic, but better than the previous recommendation overall.