From: Rabbi Moshe Sofer
To: Rabbi Zalman Leib Segal
The Chasam Sofer rules that the father's and son's obligations in Pidyon HaBen are two independent mitzvos, and redemption performed during the son's minority is of doubtful validity. He holds that agency (shelichus) does not apply to Pidyon HaBen, and rules that Beis Din should redeem the orphan using a berachah without Shem u'Malchus, with a note of 'safek nifdeh' placed on the child, who must redeem himself again upon reaching majority.
Source: Shu"t Chasam Sofer YD §295