Toddler help..

I have a step son ish... I’m engaged to his dad and pregnant with his sibling so its basically a step son :) anyways he’s 2, 3 in July. My fiancé and his ex do noooot get along and while I don’t know if it correlates, he’s from teen pregnancy, his mom is 19 now. Now he’s kind of, for lack of a better word, a raging jerk. His mom is using paw patrol as a baby sitter and letting him run her house.

He comes to us and is just mean and throws tantrums for every single thing. I know tantrums are normal for his age but this is a result of being allowed to call the shots at his moms. We’re trying to fix it but I’m not allowed to discipline him because I’m not his parent (according to her) so I can’t help too much. I’m trying but it feels like a losing battle and I seriously need help. I don’t know what to do. Today he screamed because he didn’t get enough gummy worms in his frozen yogurt, then he couldn’t keep the rocks from the store plant, then becauseI moved my seat so he would stop kicking, and so on. Please any advice, any tips, anything is greatly appreciated.

Just to add. His dad is scared to bring it up to his mom because she will say “hes an angel with me, what are YOU doing wrong??” And theyre fighting for custody rn, so anything that they can use against him is too much for him to handle. Help :((