If his intention is to call for Islam by telling the truth and informing them that Jesus is not the son then it is permissible,
But if it is just to say things that please them then it is not permissible.

Is it permissible for Muslim clerics to give lectures in church if the church asks them. a church invited a sheikh to do a lecture about JESUS AND MARY IN ISLAM, people said hes wrong to got. is it okay for imams to do lectures in Church.