What are the implications for a man who broke his fast last Ramadan after masturbating, believing that breaking the fast was obligatory to make up for that day, unaware of the necessity of expiation, and who now feels guilty?

Congratulations on being granted the success to repent. What you committed, which is masturbation, is forbidden during Ramadan and otherwise, but it is a greater sin in Ramadan. If no semen was discharged, then your fast is valid. However, if semen was discharged, then your fast is nullified, and you must make up that day, and you must refrain from eating and drinking for the rest of the day. You must sincerely repent to Allah and make up the day you broke your fast.