What is the ruling on the fast of one who involuntarily experiences a wet dream after Fajr during a Ramadan day, and is expiation required of him?

Masturbation is the deliberate expulsion of semen by any means, and whoever does so must make up for that day. However, if one did not cause its expulsion, then their fast is valid, and there is no make-up day required for them. As for expiation (kaffarah), it is only required for those who deliberately engage in intercourse during the daytime in Ramadan.