The agent of an infinitive (when it is expressed) will be in the accusative case (GGBB p192; Robertson).
Καὶ καθὼς Μωϋσῆς ὕψωσεν τὸν ὄφιν ἐν τῇ ἐρήμῳ οὕτως ὑψωθῆναι δεῖ τὸν υἱὸν τοῦ ἀνθρώπου, (John 3:14)
In this verse, υἱὸν is the agent of the infinitive ὑψωθῆναι.