Has your son tried running a bottle or two of Dishwasher Magic or Finish Dishwasher Cleaner through his machine to see if it will clear out the build up? Might be worth it . I have used it before but I have never had build up that bad before. I think Lemi Shine is also good for build-up also.You can even run the Lemi Shine with the dishes to get the gunk off of those. All this is available at Wal Mart.
In fact, it would probably benefit you also to run one of the above through your machine to clean out the pump,pipes,etc.