El perro de él
why in the sentence "El perro de él.", it is used de él instead of del?
This is because the word 'del', is a joining of the words 'de' and 'el', 'de' meaning 'of' and 'el' meaning 'the'. However, in this context the word del cannot be used because 'él' means 'him', and thus the word del cannot be used.
The spelling also highlights a different pronunciation. It's subtle, but if you listen carefully you can hear a native speaker break "de él" into 2 syllables, with the second syllable having slightly more emphasis.