Maybe he is just doing really big wees all the time - that can happen with any gender baby. There are inserts like a pad that you can get to put in them but they are quite big and are better suited to older babies. It's hard when they are so little and even newborn nappies seem big kwim? So I don't think you are doing anything wrong. Actually you could probably try even putting a normal sized sanitary pad in his nappy to see if that absorbs the extra before it can leak out?