as u says that ur son is dry during night before this episode, this may be due to more likely urinary tract infection
(UTI) or les likely due to sound sleep that he can`t felt sensation of fullness of bladder. to rule out UTI give urine sample for microscopy & for culture. go for ultrasound
to find out any other problems like stone.
meanwhile give no water in night time, no tea, coffee Or cold
drink in evening. ask him for urination just before go to bed. other treatment will be given according to reports.