What Should I Do If My Toddler Is Not Speaking In Complete Sentences Or Communicating Clearly?

My son is 3 years old but he has not started making complete sentences. He is not communicating. Though he tries to but no one seems to understand what he s saying. We have been advised by a doctor to buy encephabol syrup for him, send him to school to get his brain well activated. Please advise me on what to do.