Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
You should not inform this young man who proposed to marry you of the past sins you committed. Shariah recommends that the person covers himself as Allah likes people who-after committing such sins- cover themselves. In addition, you should renew your repentance over and over again. Allah is, indeed the Most Merciful, the Most Gracious.
Now, if the young man who proposed to marry you is pious and of good character you should accept his offer. The Prophet (Sallallahu Alaihi wa Sallam) said: "If a man whose religion and character please you proposes to marry one of your girls or women; you should marry him. Otherwise there will be much temptation and perversion on earth." [Al Thirmidhi].
You should not fear what the man who committed a sin with you might say. Indeed, Allah will protect you and take care of you as long as your repentance is sincere. The Prophet (Sallallahu Alaihi wa Sallam) said: Comply with Allah’s commands, Allah will protect you. [Ahmad and Al Thirmidhi].
Finally, we advice you to pray Istikhara before accepting this after and to respect the conditions of valid marriages in Islam.
Allah knows best.