Něj = him

This come after a preposition pro něj /něho = for him (něho is more common).

Něj is in the Accusative case. The reasoning for ně is because it comes after a preposition and it means them.

Chápeš? Do you understand?

That would be the future tense, and the English sentence is in the present. The po- prefix converts this verb to its perfective mate, and the present-looking form of a perfective verb has the future meaning.

But because we do not teach POSTARAT SE, and even if we did, it would come much later in the course, after we show the "verb aspect", I hope this exchange does not confuse those students who do not use outside sources.