First off, let me say, that those slippers are the cutest things I've ever seen!! LOVE. them.
When my DS was that age and he was wearing Robeez (or the knock off brand), I always put him a size a little bigger so that they could last longer and leave room for socks, too - he was in size 12-24 months when he was 10 months old. So maybe give those a try? But he's also almost 3 years old and wears a size 9.
He's always had big footsies.