When to Start: Seventh Generation

9 Month Old Waking up Soaked Every Morning

My 9 month old son is waking up every morning between 4:00 and 5:30 just soaked, all up the front of his pajamas. We have to change his diaper and his pajamas while he screams, and sometimes his sheet, too. I'm using Seventh Generation diapers in a size 4. He weighs about 20 pounds. I'm sure part of the reason is that he sleeps on his tummy all night. Does anyone have any solutions?? Should I get an even bigger size? Is there a more absorbent diaper? I use cloth during the day, but I switched to disposables at night because of leaking, but...