Just stick with your way and stay consistent. Kids learn pretty quickly that there are different expectations at different places or with different people. Him acting up when your mum is around doesn't destroy all your hard work. You are still his number one influence.
DS is only 21 months but he's already starting terrible two tantrums. I've been using 'time in' and he seems to really get it and is fairly easy to calm down. I learnt the technique in a parenting course.
It's basically helping the child to organise their feelings (which they have trouble doing), by the parent labelling the emotion the child is experiencing and calming/reassuring them. For example - 'I can see you're angry because I packed your toys away. Its ok to feel angry. We have to pack your toys away because its bed time now.' Plus some cuddles/hugs if the child tolerates it and the parent staying calm/kind and sitting with them.
It's quite a hands on technique so it's probably not for everyone but worthwhile looking into if it interests you
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