S.G.
C., I have a like issue with my 4 year old. If he gets up around midnight, usually the rest of the night is ok, unless he drinks after 6. He cries so hard when he wets his underwear that I have also resorted to pull ups. (Besides, changing him and the bed in the middle of the night is so tiresome.) He does not really like them, and sometimes they are dry, sometimes they are not.
I think their little muscles are just not quite ready yet! It will come in time. You are doing everything right...I think maybe his body is not ready yet.
My mom keeps telling me to use an essential oil called Cypress on his feet at night...guess this has worked for her friend...but I have yet to try it!