The translation su agrees in number with the noun which it modifies. his is translated by su, sus, according to what follows: his father/mother su madre/padre; his books/magazines sus libros/revistas. For his used with parts of the body,
- (sing) su (pl) sus his son/daughter su hijo/hija his sons/daughters sus hijos/hijas it's his house, not hers es la casa de él, no la de ella he broke his arm se rompió el brazo
The translation suyo reflects the gender and number of the noun it is standing for. his is translated by el suyo, la suya, los suyos, las suyas, depending on what is being referred to.
- (sing) suyo, -ya (pl) suyos, -yas all that is his todo eso es suyo or de él his is blue el suyo/la suya es azul, el/la de él es azul it isn't his, but hers no es de él, sino de ella his and hers para él y para ella a friend of his un amigo suyo or de él that habit of his esa costumbre suya, esa costumbre que tiene