Let it go for now. Strip every "big boy" priviledge from him, put up a chart with the things he deems as big boy, like certain toys, etc. Have him earn them. If there is some big boy toy he is wanting that has to be the ultimate of when he goes for a solid month without any issue. For my son it was a Transformer.
Put him in pullups, firmly but matter of fact like say well I guess you aren't ready to be a big boy, let me know when you are ready.
Then leave it alone. When he wants to do something his older brother does, you need to remind him he has chosen not to be a big boy yet, that big boys go on the potty and leave it alone.
He will do it when he is ready and motivated enough.
For my daughter a call from Santa is all it took, for my son wanting a Transformer was it. He wanted one and I told him they were for big boys and he had to go on the potty every single time by himself and be in underwear. It took two weeks, no joke.
He had to decide when he was ready.
If he has a favorite character think about someone from outside your house to call him and talk to him about how proud they would be (acting as the character), otherwise, cut short his bedtime maybe by 15 minutes saying he has to earn his big boy bedtime back.
Just let him decide when he is ready as you cannot force him.
Good luck, don't make it a power struggle, just put him back in pullups to lessen the stress and let it go for a while, he will come around, promise.