How do I approach my younger brother about his friendships?

Salam everyone, I could really use some advice. My little brother, who's only 13, seems to be getting involved with friends who aren't the best influence. They've been discussing topics that are inappropriate for someone his age, which goes against our Islamic values. As his sibling, I've always tried to teach him about what's halal and haram, and he knows I occasionally look at his messages. I recently noticed this in his group chats and want to talk to him about it. How can I advise him in a way that doesn't make him distance himself or start hiding things even more? I don't want to push him away, but I feel responsible to guide him properly as a Muslim brother/sister.