That text in Romans 11:29 is used by Paul in reference to His choosing of the nation Israel, so it probably does not relate directly to the matter of spiritual gifts (thus, Israel's future deliverance and faith in Messiah is sure). While I am convinced that God empowers every believer with a certain gift or gifts (1 Corinthians 12:7-11; Ephesians 4:7), I am not convinced that our spiritual gifts remain static (just the way we received them at the time of our salvation). I can conceive of God giving a believer a particular gift at a later time, when it is needed by the church. As to whether a gift is removed I cannot say with any certainty. But I can say that there are diversities of ministries and diversities of effects (results), as taught in 1 Corinthians 12:5-6. This means that the same spiritual gift may be employed in different ways, and with different degrees of success. Thus, God may give a man the gift of pastor-teacher, and that gift may for a season be used in the context of full-time ministry. But the same gift could also be utilized in a wide variety of ministries that are not full-time. When you look at Paul's ministry, he worked to support himself on some occasions (Acts 18:1-4; 20:33-35; 1 Thessalonians 2:7-12), and he devoted himself fully to ministry at other occasions (Acts 18:5).
The bottom line is that I think a man's ministry may change. There may well be other factors or reasons for this man's resignation, and they should probably be explored. If there were some matter of sin involved, stepping down would be a good thing (1 Timothy 5:19-20). If he is discouraged, he may simply need encouragement (2 Timothy 1:6-7).
Personally, I would not say, "once a pastor," always a pastor (when used of a position in the church). And I would not use Romans 11:29 as a prooftext.