How can I support my teenage brother through this?

Assalamu Alaikum brothers and sisters, A bit of background so you get the picture: I'm 20F and my younger brother is 14. Alhamdulillah we generally get along, but like any siblings we have our fights. Over the last two–three years he's become more disrespectful and, now that he's a teenager, I worry it may get worse. I used to snap back and things would escalate, but Alhamdulillah I'm trying to be more patient. About a month ago our father passed away unexpectedly and I'm struggling to find my footing. I need to be a support for my mother and brother, and right now helping my mother means being there for him. He grieved openly for the first few days up to the burial, but since then he's gone back to his usual self. I don't feel like I've made myself into a safe space for him to open up, and even if he did, I'm not sure what to say. On the positive side, he's become more affectionate toward our mother and relatives. Still, teenage years are risky and I'm worried about him falling into haram habits like zina, drugs, or vaping. He's almost always on his devices, and I discovered he was sexting girls when he was 12–13. He promised to stop, but I'm afraid he might repeat it and I admit I have distrust. The neighborhood has younger kids vaping and smoking, and I don't want him to be influenced. When I try to talk to him it often feels like it goes in one ear and out the other. He seems not to understand or remember what we discussed. For example, I explained how much it hurts when he ignores and disrespects me; he apologized then repeated the same behavior a few days later. During conversations he's often glued to his phone, and if I ask him to put it down he jokes around or pretends he’s recording me until I walk away to avoid saying something I'll regret. It's hard to stay composed. Because he's a boy, I feel his emotional and hormonal changes are different from mine. I haven't struggled much with lust, smoking, or those kinds of temptations, so I don't really know how to guide him. I want him to turn to me and I want to be a better sister and friend. These teenage years are crucial, and after our father's death I don't want him to drift away. Sorry for the long rant and if I'm all over the place. Jazakum Allah khair for any advice on how to handle this-how to be a supportive sibling, how to set boundaries firmly but lovingly, and how to help him avoid harmful influences while grieving.