No doubt your pastor has prayed about this and sought God's will concerning his future -- and if he truly believes God has led him to take this step, then you should be supportive of his decision. Even if he did decide to stay, eventually you'd be forced to find someone to take his place.
No one had a greater impact for Christ than the Apostle Paul -- and few ministers of the Gospel were more deeply loved. But even the Apostle Paul knew his work must come to an end: "The time has come for my departure" (2 Timothy 4:6). He knew he wasn't indispensable, and he was confident that God would raise up others (like his young friend Timothy) to carry on the work. May this be your attitude as well.
Thank God for the faithful ministry of your pastor; may your congregation let him know of their love and respect. It can be very difficult to be a pastor, and I'm afraid some congregations fail to appreciate and honor those God has sent to minister among them. King Saul was far from perfect -- but David rightly said, "Who can lay a hand on the Lord's anointed and be guiltless?" (1 Samuel 26:9).
Seek God's will for your church's future -- and encourage others to do the same. Then trust Him to guide you to the person He has chosen to lead you in the future. When he comes, welcome him and pray for him and do all you can to support him.