Does anyone know why he would do this? How can I correct this?
Did he miss out on experiencing these things during his critical period....before about 5 months of age? When a dog or any animal is a baby, there is a certain window of time where seeing novel things are more of a curiosity than a fear. Later, when that window closes, novel things make them very cautious. They can be curious or cautious, but if in the wild, they were curious too many times and proven wrong (that the thing was really dangerous) they would not live to reproduce and the species would go extinct. So, caution is the default behavor in adult animals or young animals who are past this critical stage. Caution takes precedence over curiosity.
All animals work off of a thing called flight distance. They need to keep a safe distance from scary things. There are two ways to do this: Either be able to take flight (run) or to make the scary thing go away (aggressive display). If the first plan doesn't work because the dog is on a leash or for some other learned reason (that flight never works because it is interrupted) then the dog goes to the 2nd option....aggression.
Socialization to many, many novel things before 5 months of age is so vital that it's impact is with the dog forever. If something proved to be very frightening, then it is very hard to change his mind. If he never experienced the novel thing in a pleasurable way, he isn't likely to be non-reactive to it. However, the more novel things a dog gets use to as a pup, the more he is able to tolerate novel things that he never saw before as an adult. All dogs vary with how much socialization they need.
Your dog's reactiveness is probably based on fear of the unknown and scary thing. He is trying to make it go away, to increase the distance between himself and the "dangerous" thing.
What I recommend you do is get a hold of a skate board and set it upside down in your living room. Sit in a chair. As he approaches it curiously (because it won't be so scary in this context), toss him a high value treat. Don't pay attention to him otherwise. Just sit there and watch him. After a few times, turn it over....do the same. Then put one of your feet on it and move it slowly back and forth. As long as he is calm, drop him a treat and praise. When he's doing well, put it away till next time. Gradually, take it outside and do all that out in your yard. Ride it a few feet yourself. If he reacts, ignore him. If he is relatively calm, toss a treat. (a good treat)
The bus and other loud noises: Take him somewhere where he can see and hear a bus but from a good distance away....whatever it takes for him to be non-reactive. Treat/praise as long as he is calm. Do this every day for a week or two until he is very comfortable with that. Then close your distance ever so slightly and do the same. It's a process and won't happen over night so have patience. Go slowly. No force, no added scariness, no scolding for reactiveness. If you get too close too soon and he reacts, be calm and turn him away once he has settled down, not before. And start from where he was successful.
Practice too, with other tings which aren't quite as scary or noisy. Set up the situation in as simple of terms as you can, as with the skateboard. And work up. Do not associate any added stress or tension with the thing he's already reactive to or highly aroused. You want to reduce his physiological reaction as well as his psychological reaction. So, keep yourself calm, unconcerned, don't anticipate and tighten up on the leash if you can help it. He must see that you are comfortable with all these things. He can read you like no other, so make it into a fun time, not a worried time.
We've tried many times to try to get him to stop but no luck. Does anyone know why he would do this? How can I correct this?