As a sponsor, you have your own ministry to fulfill

Confirmation, like Baptism, seals a Christian with an indelible mark. Perhaps you are afraid that he lost sanctifying grace and is in a state of mortal sin? That does not make him unfit to be a Catholic, rather, that simply means he is as every other fellow Catholic that sins, then with God’s grace repents and through Reconciliation receives grace again.
Repentance is the matter, here. While the Holy Spirit will try to enlighten him with his auxiliating grace, we know well that God wants our collaboration, He wants us to be lights in the world. As his sponsor, you are definitely one of the people at the top of His list for helping him to understand and repent. You hold a moral authority over Him. You know how to exercise it: being kind, like Jesus, and understanding; being open to understand his reasons - unless he has lost his mind, he must have good reasons, or believe he has them, to act that way - and then lead him to a deeper understanding of his faults.
If you see something that may constitute mortal sin, urge him to confession and let Christ work wonders in his soul. Perhaps he is unaware of his behavior being sinful…it may be evident to you, but passions do blind us…
Be patient and merciful…hate the sin and love the sinner…and realize that he needs help and love, regardless of whether he deserves it or not in your eyes.