Is you son stressed out about anything such as school? Stress can cause sudden bed wetting in children who have been dry at night before.
A cool room can also cause a child to wet the bed. Make sure he isn't getting too cold at night.
Wetting the bed is pretty normal in children and is usually caused by the bladder not growing as fast as the rest of the child's body.
If the bed wetting continues you might want to take your child to the doctor to have some tests done just to be sure it's nothing more serious such as a UTI urinary tract infection or bladder or kidney problems.
Your child's doctor can prescribe some medication to help slow down your child's urine production at night.
In the mean time maybe get him some youth diapers to keep him and his bed and clothes dry.