What is the ruling on someone who is addicted to masturbation and wants to repent, and what should he do if semen gets on his clothes?

You must hasten to perform ghusl (definition) and prayer. There is no need to purify anything that has been touched by semen, as it is pure according to the preponderant opinion. However, if you wash it, that is good. You must offer sincere repentance for the sin and not despair of it, for the door of repentance is open. If you repent sincerely, you will be as one who has not sinned. Allah is capable of warding off the harm of the act. Do not tell anyone about your sin.