Learn Portuguese (Portugal): O rapaz trouxe a mochila dele.

Yes, trouxe is the Pretérito Perfeito (simple past) of trazer (to bring) for the he/she/it/you form. Trazer is highly irregular in the past tense. Instead of adding a standard ending (which would give a fake word like "trazeu"), its base changes completely. The past forms are eu trouxe, ele/ela/você trouxe, nós trouxemos, and eles/elas/vocês trouxeram.

How do you pronounce the letter x in trouxe?

In trouxe, the x makes an ss sound. The first syllable sounds a bit like the English word "throw" but with a t instead of th, followed by an ss sound. While the letter x can make several different sounds in Portuguese, it is always pronounced as ss in the past tense forms of trazer.

Why does the sentence use dele for his instead of sua?

While sua can grammatically mean his, hers, or yours, native speakers mostly use it to mean yours (when speaking to someone using você). To avoid confusion about whose backpack it is, it is much more common and natural to use dele, which literally means of him (a combination of de + ele).

Why is the word order a mochila dele and not dele mochila?