K.,
Welcome to mamapedia!
Every child learns and develops differently. YOU know your son. If you are concerned? Take him to the pediatrician and see if he needs a speech therapist.
I wouldn't harp on him over it - like draw attention to him about it. It could be a phase, I don't know. I don't recall my boys stuttering. My youngest was always VERY clear with his words. It was funny - but we didn't laugh.
I would reiterate the word to him after he says it. "did you want your truck?" or "do you want mommy to put your shoes on for you?" Don't correct him, just say it correctly. Do you get the difference?