What's Wrong With Him???

I am worried with my 2nd son behavior and condition for the past few days.. He was a very energetic and active boy before this.. He can't seem to sit and stay calm in one place for few sec and he likes to touch and observing things around him..He likes to ask thousand question about everything he saw that it's really irritated me sometimes with his continuously question.. However , for the past few days  he is so quite and weak..

I asked him what's wrong,  but he say nothing.. I even asked him whether he feel unwell but he say he's fine.. I am out of clue what is wrong with him ... I feel something missing without his babbling and thousand question.. There must be a reason for his change of behavior.. Maybe i missed something and i need to find out why he behave like this..

His present attitude really make me upset with myself.. I don't know what to do so that he can return to his normal self.. I don't like seeing him like this... Our home feel  like not home without his story and laughter..