The dysfluency results from their talking abilities developing faster than the language centers of their brain. Once your little guy's brain catches up to his mouth, the stuttering will disappear. In the meantime, I wouldn't make a big deal out of it. He's too young to be very bothered by the stuttering, and if you get anxious about it, so will he, which may just make it worse. Continue to talk to him in a nice, slow, relaxed way and patiently wait for him to make his points.
Boys are about three times more likely to stutter than girls.
Stuttering is probably, at least in part, an inborn, genetic problem. And although there are lots of theories, nobody really knows what causes stuttering. Personally, I usually don't worry about stuttering unless it's still happening after the age of 4. The stuttering occurs in all situations, not just when the child is excited or nervous. The repetitions are very frequent and very long.