No.6 Discussion Board

Ah, no problem xD It wasn't even my work; I just translated it kinda horribly.

Well, considering that "sh" is hard to say up to their third year, I'd say he'd probably say "ch" instead just like Shion. Otherwise, it seems to be individual how difficult children find words with "s" and "sh" so you could probably explain that Nezumi can pronounce "Shion" just fine with him just being talented and a few months older (which would probably explain why he'd have no problem with the "o" either).
He could say "Shin" though xD Or something like "Shi-in" or "Shi-an" or whatever when he just decides to purposefully say Shion's name wrong because he can't pronounce it right either way (but like this, no one is gonna guess he just can't pronounce it but rather think he just doesn't want to say it right and that would be very Nezumi - especially if he keeps it up when he finally can say "Shion" but has fun seeing Shion's reaction or something.)