Step 1: Check noun number. "Trouser" in English is always used in the plural form: "trousers."
Step 2: Problem. The sentence incorrectly uses "trouser" (singular).
Step 3: Rewrite. The correct sentence should be:
"He was wearing shabby faded trousers which he explained, were called jeans and cost a fortune."
\[
\boxed{\text{Wrong word: trouser}}
\]