3 year old how to break the habit of putting fingers and items in his mouth?

My 3 year old brother got the habit of putting things especially his fingers in his mouth. His hands are always wet with saliva. I tell him non stop all day to take his hands or objects out of his mouth. But right after i tell him he goes right back to doing it. Hes going to be four soon and heading to headstart in september and i dont think its very sanitary for him to do that especially at school where everything he touches other kids will too. He only got this from his 13 year old bro who does the same thing cuz of his braces. How do we help him break the habit. He does it constantly. Watching tv, eating, playing, when he gets yelled at, shopping cart seat belts toys clothes etc. Help please and thank you!