Heavy pee at night

So my son has always been a heavy wetter. We were having issues with him being wet every morning (diaper leaking), so we went with one size up diaper, with a plastic cover over at night. That worked for a while, but now we are back to wet every morning. We tried one brand of overnight diapers, but they gave him a rash, so I haven't tried any others. Anyone have any suggestions? 18 months, weighs about 30 lbs, wears size 4 diapers during the day, and 5 at night. We use pampers baby dry.