This is just my personal experience with Summer Alumni. If there is a Summer Alumni Program available the applications will go up in Jan around the same time as the Fall program application. But be aware that they do not always offer it. It was offered in 2011 but not in 2012.
Also the only roles that are available are Housekeeping, Attractions, Quick Food and Beverage, Merchandise and Custodial. If you son was in one of those roles it is possible that if he is accepted to the same role his manager may be able to get him statused at his old location, but there is not a guarantee.