Has he been wearing them without socks? I always wear mine with socks.
Are they the right size? The foot should not touch anywhere but the bottom of the shoe.
I have several different pairs of fake Crocs and the only time I got a blister was when I brought them to WDW without checking the tread on the bottom. I managed to go through both pairs. They were used on previous trips so it was my fault for not checking. They almost had holes in the bottom.
I ended up buying at pair at WDW, wow that was expensive. I have one brand new pair of fake Crocs that will come this year.