Why is my dog like this?

He is a St.Bernard. He is very nice in the house, But when I walk him he is aggressive towards other people that walk by,He's totally different. When he gets loose he jumps on people and growls at them, never once did he bite anyone though. My mom said he still thinks he is a puppy and is just being playful. I think he is just being vicious. Aren't St.Bernards suppose to be nice? like beethoven.