The problem here from my view was your mother. If she is setting him off like that, she needs to be told firmly to stop, if she cannot stop, she needs to leave. You don't need to prove to her how your son will react, she needs to respect you and your parenting decisions. Sometimes when DD is really violent it's all I can do to keep her in a safe place too, but the calming tools are of course better given you have the chance to use them.