2 1/2 year old not talking?

My son will be 3 in 5 months and still isn't talking much. He is very intelligent and doesn't show any delay in anything else. He does speak a little he will say thank you, please, snack, hot, cold, don't want to, let go, and no. He will also call my husband Dad but will not say Mom yet. He generally just uses baby babble and or yelling when he needs something. I challenge him to say the words but he won't. I have never ever gotten him to say something on queue. I'm a stay at home Mom and I am very interactive with him. I read him stories I even do the your baby can read tapes with him. I am watching my cousins 2 boys today her oldest is 3 months younger then my son and speaks in full sentences. I feel there may be a delay. Any advice or experience would be great.