How do you say his in German?

"sein" or "seine" depending on the gender of the noun that follows.

Sein Buch - his book (n)

Sein Hund - his dog (m)

Seine Frau - his wife (f)

Seine Kinder - his children (pl)

Depending on the case (genitive, accusative, nominative or dative) the endings could change to seines, seinem, or seiner

Das Buch seines Sohnes - his son's book

Er ging mit seiner Frau spazieren - he went for a walk with his wife

Er spielte mit seinem Sohn Fußball - he played football with his son