My child is 2.9 years old. He is very active and restless. He understands everything but can't respond with even a word. He can say just 1 or two words, such as water, car, toy, give me, etc. After pushing a lot, he may say something if he wants to. He spends the whole day watching cartoons on TV. In fact, he has stopped saying the the words he used to say as he prefers action language rather than talking. Should we take him to a specialist for speech therapy?
Yes, you should take him to specialist for speech therapy.
Or, rather, take him to a specialist who can determine what may (or may not) be the issue and if speech therapy is the next step.
Also, maybe lay off the cartoons for a bit. That much TV for a developing mind isn't a good thing.
Turn off the TV!
Sorry for shouting, but a toddler should not watch much television, and certainly not "all day" as you state! Our user dave posted this very related answer:
It's no wonder that he doesn't want to talk - you're training him to be a media consumer rather than an independent individual.
Our user Rory has another great related answer: you should be his entertainment, not the TV. That page has even more good answers you should look at.
By letting him watch TV all day, you are overloading his brain (because small children cannot mentally process so much media). That might also be a reason why he is so restless. By letting him do this instead of playing with toys, you are hindering his creativity and independence. He needs to learn that he can find an outlet for his energy and restlessness by doing things himself — in a sense being a producer (of creative stuff) rather than a consumer (of TV).
If he's been good during the day, you may want to reward it with five minutes of TV as part of his regular bedtime routine, but make sure to show him something that is low-tempo and age-appropriate. For instance, most Disney movies, and shows on TV channels like Cartoon Network, are not age-appropriate to a toddler! An example of good toddler material would be one 5-minute episode of Barbapapa or similar — there's a reason why these episodes are so short and (to an adult) rather boring. That's age-appropriate.
Update: Should you take him to a specialist for speech therapy? Depends.
In addition to turning off the TV, and possibly checking in with a specialist if things don't get better pretty quickly, it will likely make a huge difference if you increase your verbal interactions with him (something TV watching usually stifles). Since turning off the TV is likely to create a situation where all of a sudden he doesn't know what to do with himself, you'll have to start "entertaining" while also getting stuff done. It may not feel like it, but this is a good thing.
Things to do with him that Increase Your Verbal Interactions:
While these activities will increase your verbal interaction with him, there is still one more piece of advice I have for you. He's old enough, and it sounds like you know he is both physically and mentally capable of more than he is doing now. Because of that fact, don't accept "half speak."
If he says, "Give me!"
Then, after about a week of this kind of exchange has occurred, require him to say the entire statement (or at least attempt it) before he gets the ball.
I wouldn't recommend this part normally, but it sounds as though there is some back-sliding that is needing remedy and this kind of an attitude about it will help. Of course, this part may not apply if there really is some sort of physical or developmental delay in addition to the TV problem - that is what a specialist is for determining.
On the rare occasion that you do watch TV with your child. Treat it more like a book. Sit together and watch something "age appropriate" and designed for toddlers. Pause the video frequently and speak with your child about what just happened or what you are seeing. Ask him to describe what he sees, likes or doesn't like about what he just saw. As he gets older (and the shows become more complex) you can also ask for "predictions" "What will Pokoyo do next?" - "Will the butterfly go away if he makes noise?". . . (Pokoyo may be a good one for you to check out now as a TV option for your child actually). In the linked example, a good question might be, "How does the narrator know Pato wants to be the postman?" Just before he takes off to take Ellie the post. (He could also predict when the narrator asks questions (that is the whole point of having those bits in the program). It is translated in many languages by the way. Even interactions like this should be limited, but occasional TV viewing is okay if it is an interactive experience for your child - that means you have to be there and talking about it together while you watch.