My brother is 12 and was staying with a friend when our father was on the late shift. He called and asked me if Papa was there. I said "no, you know". Strangely enough, he came home shortly afterwards and smelled of cigarettes. I asked him why he smelled like smoke. He said he didn't know what I mean and he took a shower straight away. Then I asked him why he called, he said it was none of your business. Now a few days later I saw by chance in my history that he googled on my laptop how high the penalties are if someone sells cigarettes to children. I asked him about it and he said it was for school and I should stop asking him, he has also become really aggressive.
Nice that you are worried about him.
Include your parents. He doesn't seem to be listening to you and it is important to act immediately if he really smokes.
Tell him how it will ruin his life if he continues to do it.
If he doesn't stop, talk to your parents about it. Your brother won't be happy about it, but you and he will be happy later that you did it.
I think it's good that you worry, your brother should consider himself lucky that you think of him.
Am your brother's age myself and will hopefully never start smoking (everyone says yes, but at least I hope). Talk to him Tacheles that you know he smokes and Google for shock images and quick consequences, e.g. Bad breath or the girl is not so into it (no plan if that is so) etc.
good luck and don't even start doing it yourself:-)
To help your brother, you should tell your parents. Smoking is harmful, especially at this age. That is not how it works.
He must have smoked. Unfortunately, there's nothing you or your parents can do about it. If he is forbidden, he still does it secretly. If I were you, I would talk to him about it in peace and you should tell him that you are not stupid and that you know exactly that he smoked, because you now have enough evidence for it.