Your kid, at this age has now started speaking very well. He has a vocabulary of thousands of words to express his thoughts. He is reading books and learning more and more new words some of which are tongue twisters and others are sounds - 'th, s, sh, ch, wh' etc. He is also in contact with so many people and learns speech from them too.
What you need to know:
As a parent, you are very much responsible for the all round development of your child. Talk to your kiddo as much as you can, stressing on words you feel he is unable to speak. Give him the practice of those words. Make him twist his tongue and touch molars for pronunciation of words. Gentle corrections with wrong words need to be made. Read out story books and ask him to narrate stories to you. Also never neglect your native language. Apart from English speak your native language at home. This gives a good base for language and speech development.