How do you spell his in French?

When you want to say "his" in French, you say either "son" or "sa".

If you want to construct your sentence correctly, you must keep in mind that in French the possessive pronoun doesn't tell you the gender of the person who owns the object, but the gender of the object itself.

In English, when you read the sentence:

"It is her Apple," you know that the Apple belongs to a female.

when you read the sentence:

"It is his Apple," you know that the Apple belongs to a male.

It doesn't work this way in French. In French, when you see the sentence:

"C'est sa pomme," you don't know whether the Apple belongs to a female or a male person, but you know that the Apple is gendered female.

When you read:

"C'est son melon," you don't know whether the melon belongs to a female or a male person, but you know that the melon is gendered male.

Things get tricky when it comes to nouns beginning with vowels, because we always use the posessive pronoun "son" for these Words, regardless of their genders. So although an orange is gendered feminine in French, you would still write:

"C'est son orange."