Is it obligatory for a young man who broke a man's head before his repentance to go and ask for forgiveness from this man, or might that harm him by reminding him of the incident?

Shedding blood is a grave matter. The Prophet, peace and blessings be upon him, clarified that the first matter to be judged among people will be bloodshed, and that a believer remains within the latitude of his religion as long as he has not shed unlawful blood.

The repentance of a servant is accepted under its conditions. If the sin involves the rights of people, then one must seek absolution from those whose rights have been infringed upon. If the crime was intentional, then if retribution (qisas) is possible without fear of the perpetrator's demise, qisas becomes obligatory unless the perpetrator and the victim reconcile; otherwise, blood money (diyah) is the prescribed amount.

The young man must satisfy the person he wronged and obtain his forgiveness by any possible means, even if it requires going to him should no other means be found.