From my experience with bulldogs two years is usually the age the fear goes out and the courage kicks in.
On a side note and related, a dog will look to you as the owner to understand the world around him. When a car goes loudly down the block act as if nothing out of the ordinary happened and just keep walking. The same thing with any loud noises. When there is thunder act like nothing and your dog will adjust to thinking there is nothing wrong.
Expose your dog to as many different situations that you can while he is a pup so when he is an adult he will be able to decipher friend from and foe.