Is it obligatory to make up or offer expiation for someone who had a wet dream during the daytime in Ramadan and was aware of the dream and able to stop it but did not do so until ejaculation occurred?

The Pen is lifted from the sleeper, so a wet dream does not invalidate the fast, even if it reoccurs. As for the emission of semen due to thoughts, it does not invalidate the fast according to the majority of scholars, due to the Prophet's (peace be upon him) saying: "Indeed, Allah has pardoned for my Ummah whatever they whisper to themselves, as long as they do not act upon it or speak of it." However, if the emission of semen was brought about by you intentionally while awake, then you must make up the fast and repent. Expiation is not obligatory in any case.