I just adopted a stray small jack russell terrier mix - He is full grown, not yet neutered.
He gets along very well with my Chihuahua (neutered male) and with my Shih-Tzu (also a neutered male).
But whenever I take them to the dog park, he is SO afraid of dogs bigger than him that he barks/growls/ shows his teeth and looks like he is going to attack! I have only taken them to the dog park twice now because I don't want him to bite a big dog, so i need tips on training him to not be afraid.

Could it be that he had a bad experience with big dogs in the past?
Also, I have heard neutering will help him be less aggressive, is this true?