Well you have to prove it and slander is when it's a lie and to be truthful your bf is not a "true" Christian im the eyes of the pastor so.he didn't really lie. But maybe you can still Sue for interfering with his job? I don't know tho good luck
Your fiance should consult his attorney to see if he has a winnable case against this pastor.
It could be problematic, since the defense against such a lawsuit would be that the statements made by the defendant are true. The pastor's statements may be opinion, not factual statements, and therefore subjective. Can the plaintiff (your fiance) prove that they are false statements?
None of the guesses from strangers online matter much. His attorney's advice matters a lot.