How do you replace a noun with a preposition other than à or de please?

Question

Il faut que j’achète un cadeau pour son anniversaire.
I must buy a present for his birthday.

I know you can use the adverbial pronoun y to replace 'à + noun', and en to replace 'de + noun',
but how would you replace 'pour son anniversaire' in the sentence above please?
In order to make the sentence: I must buy a present for it?

[Am i right in thinking this called replacing an object of a preposition with a pronoun?]

Accepted Answer

Bonjour Andy !

To replace a noun introduced by a preposition other than "à" and "de" (pour, avec, sans, devant, derrière, sous,...), you will use the pronoun "ça" or its more elegant (and a bit antiquated) form "cela".
In this case, the sentence will be:
"Il faut que j'achète un cadeau pour ça."

Though here I would suggest a better alternative would be to use another noun, such as:
"Il faut que j'achète un cadeau pour cette occasion."

I hope that's helpful!
Bonne Année !

Answer

Aha thank you.
With the precedent set by "de + a noun" and "à + a noun" being able to be replaced by the appropriate adverbial pronoun, I must say I had become somewhat fixated that a noun + one of the other prepositions would follow a similar construction (i.e. a pronoun before the verb). 
In fact the answer was a lot more simple and straightforward!
Thank you!