My boy was also using pampers for nighttime before and had the same problem as your little one - leaks. We have then tried a few brand and surprisingly Nepia works well, although it looks quite thin. His clothes now stay dry until morning. You may want to give it a try.
It's packing is in dark blue colour with Japanese character printed on the plastic bag. 1 pack is 14pcs only (not very sure) for weight up to 15kg. If you buy it from supermarket or Mannings or Watson's, it will cost around HK$53. a little bit cheaper, says HK$48 or HK$49 at 藥房, but not every 藥房 sell this one
for the case of my daughter, "pampers overnight" is a bit tight for her, therefore, I buy "pampers pant" size no. 5 which is for 40 lbs small kid. It's packing is in red colour, 20pcs in 1 pack cost HK$99, I buy it at Pedder Building 5/F at Central
I never see this one (in red packing) available in local supermarkets. Size 5 is the largest, they have size 4 also which I think is similar as the overnight one. Pedder Building 5/F is the only place I found this product.