Using throw away

My brother, (around 10-15) Shits and pisses himself on a daily basis. Every day, he will shit himself, and claim its a fart, even when we can see the literal human shit dribbling down his leg. He pisses himself too, all while wearing the exact same fucking clothes for weeks on end. As of now he has not taken a shower in 2 +weeks. Whenever he soils himself, he will go to the bathroom and ‘clean’ himself. Whenever we confront him, he starts screaming and crying, and storms of into his room like a toddler. But since he is the youngest, he gets a shjt ton of slack, and our parents let it slide.

I need help.
How do I stop this?
How do I get him to start acting his age? How do I get him to stop playing League Of Legends?
How do i get him to fucking shower?