Can Parental Alienation be grounds for changing child custody?

My ex’s father is alienating my son from me. He says terrible things about me and my current husband to my son and does anything he can to turn him against me. My son will now not listen to anything I have to say or abide by my rules when he is with me. My ex and I currently share custody, but I think I need to obtain full custody and get my son away from his father’s negative influence. My son will object to this I am sure, but he’s only 12 and I don’t think he should have any say in the matter. Is parental alienation enough to warrant changing custody and how do I prove the negative things my husband is doing to my son?