ὁ Φίλιππός ἐστιν ὁ τοῦ Δικαιοπόλιδος υἱός. 
Strange genitive (born-of) form. New words and words in dictionaries are given with their genitive singular forms, so that we always know what declension we're dealing with and how to change the forms to match the meaning.
(Δικαιοπόλιδος  looks strange. According to the 2nd edition it is the genitive form of  Δικαιόπολις. But In the 1st edition it says: Δικαιοπόλεως instead. I don't know the reason for this change in the 2nd edition because, according to the Perseus site, Δικαιοπόλεως is correct, whereas no mention is made of Δικαιοπόλιδος. So I'd say, use either, according to your version of the text.)
ὁ  πατήρ, τοῦ  πατρός
ὁ  τοῦ  υἱοῦ  πατήρ  the son's father is not
ὁ  τοῦ  πατρὸς  υἱός  the father's son
οἱ  τοῦ  πατρὸς  λόγοι: the words of his father are also
οἱ  πατρὸς  λόγοι: a father's words (There is no indefinite article a, an in Greek)
 
  Note  
 
In English: the life of the man
In English: becomes: 
In English: the man's life
We dropped one the.
But in Greek we don't, so we say:
  the   of the man life
   τοῦ  ἀνθρώπου   βίος  
In text 3a we had:
ὁ  λίθος  πίπτει  πρὸς  ...
The stone falls onto ...
the foot    τὸν   πόδα.  
  whose foot?     τὸν   τοῦ  Δικαιοπόλιδος   πόδα.
 
And in text 3b, Philip is
Whose son?     ὁ   τοῦ  Δικαιοπόλιδος   υἱός  
 
 Remember this: 
 
Greek has 3 declensions (Latin has more and they are more difficult!). You've only had 2nd declension so far (words ending in -ος or neuter ones ending in -ον). Chapter 4 deals with 1st declension (feminine words ending in or , masculine ones in -ας or -ης). All other words belong to the 3rd declension:
ὁ πατήρ, τοῦ πατρός
ὁ Δικαιόπολις, τοῦ Δικαιοπόλιδος
ὁ ποῦς, τοῦ ποδός 
3rd declension genitive ending: -ος (and, in some cases, -ως)
 
This is not an exercise in grammar, so say the little patterns out loud (for your ears to hear, your eyes to see) and for your mind: think of what is meant when you say one rather than the other.

 
ὁ  Δικαιόπολις, τοῦ  Δικαιοπόλιδος
Difficult? No, only if you make it so, by thinking: oh these cases!, oh these forms!, how can I ever learn them? Easily, tell yourself:
ὁ  Δικαιόπολις sounds like he's up to something, whereas
τοῦ  Δικαιοπόλιδος  sounds like "born of, belonging to, coming from" this man.
Try saying a few things he is or does, using his name of course, since that is the point of the exercise.
ὁ  Δικαιόπολις  δειπνεῖ.  
ὁ  Δικαιόπολις  δίκαιός  ἐστιν.
(he's just, fair)
Your turn!
 
And now say a few things that are his, belong to him. They don't all have to be said in Greek, English is fine, as long as you get a feeling for the genitive form: τοῦ  Δικαιοπόλιδος. And don't think (and feel) that the ending -ος automatically means: nominative, masculine. Because it obviously doesn't!
Whose cart? τοῦ  Δικαιοπόλιδος
Whose wife?
Whose δοῦλος ?
Whose κλῆρος ?
        etc.
 
φεῦ  τοῦ  ποδός
Lesson 3a, my poor foot! Genitive form for  ὁ  ποῦς  the foot. Learn it as ὁ ποῦς, τοῦ ποδός meaning: foot. (imagine one: a big one, small one, dirty one, inside a shoe......)
Where is your right  ποῦς ? And where your left  ποῦς ?
After the stone has fallen on his foot, the colour τοῦ  ποδός is rather ugly. The Greeks would have preferred:
the τοῦ ποδὸς colour (τὸ τοῦ ποδὸς χρῶμα)
τὸ χρῶμα, τοῦ χρώματος colour: appears in quite a few English words: chrome, chromatic, chromatography...