15 month old screaming.

I can't for the life of me get my 15 month old to talk. He babbles from time to time and he understands everything we say to him but I can't even get him to say bye or hi. He says dadada and mamama (sometimes very rarely). But he cosntantly screams, when he is playing in place of talking or babbling. He can find his ear, stomach and nose. When I ask him if he is stinky he plugs his nose. What is going on with my child.... He is my first and I can't help but feel I'm doing something wrong.