Is the collar I just bought okay for my dog?

First order of business is a visit to the vet. many ailments will manifest themselves in aggression. If he is not neutered, do that first. Have a full thyroid panel done. A prong collar is fine as a training tool but you need to learn to use it properly and enroll him in training as well. Be up front with the trainer about his issues- a regular intro obedience cl is not what you need.