Potty training

Mommies PLEASE help! So I’ve finally got my son wearing underwear recently during the day. He will be 4 next month. We’re trying to potty train him so he can go to school with his brother this August. However first he would use his underwear and not go to the toilet well now he goes to the toilet to pee but he poops everywhere like in his underwear and it runs down his leg and in the carpet. This has happened SEVERAL times. He’s ruined so many pair of underwear. I’ve disciplined him so many different ways and nothing is working. What do I do 😭 his older brother is 5 and he cried 2 days when he was his age but ever since no accidents and always going to the toilet.