If he attended my church and was public about it, he would likely not be permitted to receive the sacraments until such time as he repented, nor would he be allowed to exercise any sort of a leadership role (usher, lector, choir, parish council, ccd teacher, cyo coach, etc., etc.), but the only way I could see where he would be utterly shunned would be if he did something directly as a threat to public decency and order within a Mass: for example, if he got up and started cursing the homilist as he preached a pro-life homily, if he tried to steal the Body of Christ during communion, and so on.
When I speak of excommunication, I speak of revoking the status of membership in the church... “A formal ecclesiastical censure that deprives a person of the right to belong to a church.”
That is separate from shunning, which is up to the congregants as each situation is unique and may involve complicated interpersonal relationships.