Adults often believe that there is no reason why a word is spelled the way it is. But that's just because they haven't been taught otherwise (yet). There is always a reason!
With <is> and <his> , there are two main explanations. Here they are:
Etymology of <is>:
From www.etymonline.com :
"Until 1500s, pronounced to rhyme with kiss".
One of the ways we do learners a disservice is by either intentionally (or unintentionally) teaching them 'single-letter letter-sound correspondence'. We do this, for example, by telling them that <a> says 'a' as in apple, and <b> says 'buh', as in butterfly.
In fact, there is no letter (I think) that only ever represents one sound. You can never know what sound a letter is representing (or indeed if it is representing a sound at all) unless you look at the other letters around it! For example - <t> is very different from <th>, <o> is very different from <ow> or <oo>, and <c> can represent either /k/ for cat or /s/ for city. The <w> in <wash> is doing a completely different job from the <w> in <write>. (<Wr> words are fascinating, by the way! They relate to 'twisting/turning' - e.g. When we write, we twist our wrists. We can also wrestle, wrangle, be wrong (have twisted logic), wrap something, make a wreath by twisting leaves around a wire, wrench, wriggle, wring, or go awry - and much more!)
But back to <s>
The letter <s> is very busy - it has an important function making plurals, and it also appears in base words. In both these cases, it can represent either /s/ or /z/. We just don't notice it that much when it is showing that a word is plural.
Have a look at this:
cat + s --> cats
dog + s --> dogs
What 'sound' is the <s> representing in these words?
Answer:
In <cats>, the <s> is pronounced /s/.
In <dogs>, the <s> is pronounced /z/.
The letter <s> can represent either of these 'sounds'.
Here's a diagram based on one from real spelling to show this:
In the case of <his>, it is also perfectly regular to use the <s> to represent the /z/ sound. <S> does not always have to stand for "snake"!
Well done to the young girl who knew there had to be a reason why! I hope she never stops searching for understanding.