My son 20 month old. 1. knows 24 words from flashcards and books. 2. but doesn't use them functionally. he doesn't say even mama or papa. 3. he is socially interactive, points to object for excitement, copying action 4. not understand simple command?

Have him evaluated: I would consult both his doc & a speech & language specialist. He should be further along with his communication skillset. The type & depth of problem can only be gauged with a formal assessment.

Evaluation: Your child seems to be missing some important landmarks. An in depth evaluation is in order. Start with his Physician, hopefully a Pediatrician. Your son needs an Advocate who will be willing to fight for him. Be that advocate. Make sure he sees a child Psychologist that is fluent in assessing developmental achievements.