If it is definitely true, then confrontation privately is necessary. If he won't repent and refuses any rehab, then remove him from office. If this doesn't get his attention and he still refuses to repent, treat him as an unbeliever and revoke his church membership until he repents. Restoration is the goal, but some people do not think they need it. Paul dealt with sexual sin in the church at Corinth, and he definitely did not sweep it under the rug. One last thing, if he eventually repents, receive him back in as a brother.
Paul dealt with sexual immorality in the church at Corinth. Never has there been a time when real men need to stand up and do what is called for in church discipline, lovingly, yes, but severly as well as per what Paul said:
1 Corinthians 5:1-5 (HCSB)
1 It is widely reported that there is sexual immorality among you, and the kind of sexual immorality that is not even ⌊tolerated⌋ among the Gentiles—a man is living with his father’s wife.
2 And you are inflated with pride, instead of filled with grief so that he who has committed this act might be removed from your ⌊congregation⌋.
3 For though I am absent in body but present in spirit, I have already decided about the one who has done this thing as though I were present.
4 When you are assembled in the name of our Lord Jesus with my spirit and with the power of our Lord Jesus,
5 turn that one over to Satan for the destruction of the flesh, so that his spirit may be saved in the Day of the Lord.