When watching TV programs, he likes to talk about the story to the family members, as if they are not watching the program...

We have to ask him to shut up his mouth sometimes.

How to guide him in order to stop this bad habit ?

What occupation is most suitable for him in the future ?

Sitting down with him, without the TV on, and talking to him about your concerns. Tell him that when someone is watching a show, it is impolite to talk to them, except between on commercials, or should the other person begin the conversation. If he likes to give away things about a show he has already seen, explain that it is disappointing to hear the story from someone else, rather then expereincing it first hand, as he did when he first watched it. Some people enjoy spoilers, but most don't. A lot of things will come with age and maturity. However its important to explain things to him one on one, and have a real conversation about it. Should he continue to do it even after you have discussed it with him, and gentle reminders don't work, perhaps ignoring him during the show, refusing to listen to anything he says until the ads come on, perhaps this will get a point to him. Perhaps he does it because he knows that it annoys people, and he likes the reactions he receives. If that's the case, ignoring him while the show is on, or simply not watching a show with him that you haven't seen, might get the point across. It could be (which is all too easy to happen)that he feels you watch too much TV and he doesn't get enough time to talk one on one with you. In this case, taking a hour out of your schedule, just talking without the TV or any other distraction, before or after your show will help to get everything he wants to talk about, out on the table, without conflicting with the show.

I agree with buggage, except when he refuses to listen tell him to leave the room instead of ignoring him because this lets him get a way with it. Tell him you have talked about this and there will be no exceptions. I am sure it will turn out OK! Good luck!