Every parish has someone like this. Learn to love them, and realize that, perhaps, God has placed him there to test you, and help humble you. If you haven't already, introduce yourself after Liturgy, and give him a big hug. At least he is coming to Liturgy, albeit late.
It's always a hard thing. On one hand, when a little old Greek lady told me, "Stand up, this is the most important part," when I, as an inquirer, was sitting during communion, I was offended at first, and then took it to heart. But, on the other hand, when I see other people sitting during communion, I think, "Well, maybe they can't stand." Or, if they're able-bodied, it just pains me. Maybe they would learn something if I or someone else told them to stand up, or maybe they would be totally offended and harden their hearts. I don't know. With this poor fellow, my advice would be not to tell him what to do unless you are his friend.
I remember an instructive episode from the life of Elder Porphyrios. A woman who was scantily dressed came to visit him. The Elder had a spiritual son, Mr. Niko, visiting him at the time. The woman went and talked with the elder, and Mr. Niko thought to himself, "Why doesn't the Elder tell this woman that she's dressed inappropriately?" It was a big problem for him. Then, after she left, the Elder said, "Mr. Niko, I'm afraid I am a bad spiritual father." "Oh, no, Elder! You're a good spiritual father," Mr. Niko said. "No, Mr. Niko. You saw that woman--how inappropriately dressed she was! And I didn't tell her that." Then Mr. Niko realized his error and asked the Elder to forgive him and the Elder said, "If I had told her she was inappropriately dressed at this point, it would've gone in one ear and out the other. She's not ready to hear it. Instead, I told her what she could understand and, little by little she will come to understanding and her conscience will correct her." And, later, the woman showed up more properly dressed. This also happened through the Elder's prayers.
So, you see, we need discernment, which Elder Porphyrios definitely had. It may seem like a good solution to reprove someone for his behavior, but, if you don't know the whole picture, particularly what's going on in his soul, you could do more damage than good.
If you can find a way to encourage your brother, carefully and with love, you may be able to lead him to correct himself, which would, I think, be more profitable and sustainable than correcting him.