3 yr old son talking

hi my name is pinky. i have a 3 yr old son. he knows a lot lot of words, numbers, sings all nursery rhymes, do phonics songs like a a apple, b b ball, but when i ask him questions doesnt want to talk much. he follows all my commands and does follow instructions, says hi, shakes hand when i ask him to do so. but he is a little lagging in speech or is he selective talker i am not sure. any input please?