Couple of quick questions:
Is you son underage? and
What are they pressing charges for, ie. was there are clearly displayed policy that "inappropriate material was not to be accessed from that machine as a condition of use"?
Finally, regarding the material was this "normal" adult material or of a class generally consider more offensive, ie. child related, etc.
Centre management has no obligation to block anything, especially given that adults would be using the same computers. The person responsible is the person who decides to access the material. How old is your son?
Nevertheless, I doubt your son has committed an offence by merely accessing x rated material. However, if he allowed it to be viewed by others in a public place, he may have. It would depend on the circumstances.