You can only get a urine analysis done in that case to look for any infection in that case. Meanwhile do few things. First, stop all beverage intake like tea, coffee and soft drinks completely for a month. Second, give him dinner at the earliest, say not later than 8P.M and he should not drink any water or milk or any fluid after that. Last, make him void before going to bed around 11P.M.
By these techniques, his bladder will be completely empty at night and if its plain case of Enuresis
, you will see him cured of this habit in 15-20 days. Good luck.