I can bet your son is not doing it on purpose -- had he been doing it purposely, he wud not have hidden it from you. Rather he wud do it during day time as well. I have been a bed wetter all my life. My parents too felt that i did it on purpose -- to spite them, to get the thing i wanted or because i was upset. But that was not true. Trust me no one would like to wake up wet and take care of the laudry the first thing in the morning. Your son definately needs help.Talk to him. Probably he is scared of you/ of punishment. Moreover, by telling your son that he is doing this on purpose, you are telling him that you do not trust him. As it is it is not easy for him to deal with this. It would be all the more difficult without the help of parents.