What is the ruling on the fast of one who is accustomed to fasting Mondays and Thursdays, and masturbates during his fast, but holds back the semen from exiting? And does this ruling apply to the fast of Ramadan?

You must repent from masturbation because it is forbidden. If it is done during the day in Ramadan, the sin is even greater. As for the fast, if you feel the semen moving and it is about to exit but does not, many scholars consider the fast invalidated by this and necessitate ghusl (ritual bath). However, if the semen does not move, the fast is not invalidated merely by the sensation of pleasure.