There's no actual cure for bed wetting but you can get medicine to help slow urine production in your 7 year old's bladder.
You'll have to take your 7 year old to the doctor to get the medicine prescribed and your child's doctor may also be able to suggest a few other things to help with the bed wetting.
If your 7 doesn't already you might try getting him to wear some youth diapers or pull ups to bed to help keep his clothes and bed dry.