My cousin is already 3 1/2 years old but still can't pronounce a complete word. Should his parents bring him to the doctor already?

In my opinion, yes. Ideally, here's the pattern: kids who are a year old should be able to speak at least one-syllable word ("ma" for mama). Two-year olds should be able to speak at least two ("bibi" for baby). So on and so forth. There are kids who aren't really articulate in speaking full words, but have a grasp in what those words really meant. That's a good sign. Identify if your cousin is like that. If yes, probably he is just a late bloomer. You can search for speech exercises for toddlers. But if there's no improvement, have him checked by a doctor.