Learn Turkish: Oğlun nerede? - Where is your son?

This is because of a rule called "vowel drop." In a few specific two-syllable Turkish words (often family members or body parts), when you add a suffix that starts with a vowel, the vowel in the second syllable drops out. So, oğul + -un becomes oğlun.

How does the word oğlun show that it's "your" son?

In Turkish, you usually show possession by attaching a suffix directly to the noun. The suffix -un means "your" (when talking to one person). You could also add the word senin at the start (Senin oğlun nerede?), but since the -un already tells us whose son it is, senin is usually dropped unless you want to emphasize it.

Where is the word for "is" in this sentence?

Turkish doesn't have a standalone word for "is." Instead, it uses personal suffixes attached to the last word. For the "he/she/it" form (third person), this suffix is usually completely blank! So the word nerede (which literally means "at where") naturally acts as "where is" all on its own in this context.