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?

i've read that ideally based on his age, he should be able to talk with comprehension of the words and vocabulary should be relatively wide. You may want to check these baby milestones for further read. http://www.webmd.com/parenting/guide/baby-talk-your-babys-first-words