maybe you need to go DOWN a size, toddler are quie big, DD has just gone into them at 13 months and almost 10kg, like arcadia said are they leaking out the side due to gapping around the leg?? other wise invest in a PUL cover for cloth nappies, one with gussetts should prevent leaks.

is he feeding overnight?? DD sometimes leaks from a toddler overnight, sometimes not!
what does he weigh?