Encourage your son to drink LOTS of water early in the day. The pediatric urologist suggested up to 24-32 oz. during school time if possible. It helps your child get the liquids he needs but earlier in the day, so there is less of a desire to drink later and he has had time to eliminate what he has drunk before bedtime.
Also, consider what foods might trigger his bladder--chocolate (anything with caffeine), sugar, wheat, citrus, acidic or spicy foods, carbonation, etc. Make sure he is not constipated which can exacerbate enuresis as well. And make sure he is getting enough sleep. Sometimes an overtired child will sleep deeper, not noticing when he wets.
Our son is 12 and still has nighttime wetness. Not as much now, but he still has it. For some kids, the alarms and medications help. The alarm worked great for my nephew. Unfortunately, neither helped our son. We've tried most techniques suggested below throughout the years and more. The ones that seemed to cut down on the floods the most were a watch timer to remind him to pee every two hours during the day and drinking more while at school.
For years, we used the large washable pads under and over his sheets. When he got wet, we just pulled off the pad. Or if it was a flood, the entire bed would be remade. (I never heard of the multiple pads and sheet layers till later--probably a great way to get dry fast at night!) We never made a big deal out of it, not punishing him for something he couldn't control. To this day, he still sleeps so deeply. However, a year or two ago, we said enough is enough. He consistently wears Pull-ups to bed. I'm not sure what we'll do when he outgrows the largest size. But I've been told, if nothing else has helped, often puberty is the final solution. Here's hoping!