Why isnt my two year old talking?

He will be 2 next month and isnt talking much.I see kids younger than him and they are talking alot. Before he was even 1 he would say things like dada,mama. He didnt know what the word ment but he was saying it. He would point to flowers on the bowl and say "pretty". Then it just stopped. He doesnt let me read to him but will sit by himself and flip through the books pages. He says no,go, go away, stop it. He says "gankgoo" and thats his thank you but he use to say "giggoo". Sometimes it'll sound like he said something but never says it again. Everyone said they heard him say "Come on in" the other night but i dont know how he would of even learned that and he hasnt said it again. Even though he doesnt talk much he is still very expressive. He uses he hands alot and will just start yelling "ahh da da da dooooooo" and stuff like that. He is a very hyper and energetic kid. Im just kind of concerned.