I agree with what you were told initially - prong collars are better than slip (choke) collars because the weight of the dog against the collar is more evenly distributed around the prong collar than it is on a slip.
As with ANY training collar, prong collars need to be properly fitted and used only under the guidance of a qualified trainer. Too many people use training collars incorrectly, which is what the true cause of cartilage/neck damage, not the collar itself.
If your dog is being allowed to pull on the lead regardless of the type of collar you have him on (yes, even a flat, nylon buckle collar), you run the risk of damaging the cartilage in his trachea. If your dog is properly taught not to pull on the lead, there is no risk, regardless of the type of collar he has on.
Best course of action would be to enroll him in an obedience class and ask your instructors how to teach him to walk properly on a loose lead.
Good luck!