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According to my reading of Can. 915.
It does not apply since there has been no declaration of excommunication or of interdiction, therefore Can. 915. is not in force.
Can. 916 might apply though. It depends on how grave the sin is to disagree with ones local ordinary on matters of faith and morals, or it might hinge on how grave and public his "active" support for homosexual marriage is. I am no canon lawyer though.
If he said it was his priority and sponsored it, seems clear to me.
He’d better hope I don’t see him in the communion line, unless he has his arms crossed for a blessing.