I don't have time to look at the responses first, so forgive me if I'm just repeating what others have said.
I read somewhere that many kids go through a point where their brains are moving just a little quicker than their language development can keep up with. Most kids who start to stutter at a young age are just going through this phase and it will pass, as their minds and bodies get in synch.
My son has always been very articulate too, and went through a brief period where I noticed he was stuttering a little bit and there were hangs, mid-sentence, where he would seem unable to find the word he needed to continue. It was all I could do to keep from finishing his sentences for him, and felt frustrating. We made it through though. I don't remember how long it took, but he's almost six and has been speaking wonderfully again for quite a while. Another sign of it was repeating a word over and over until he could move on to the next word. My daughter did more of that one. She's three now and also speaking just fine.
I wouldn't worry about it unless it persists. Keeping your patience while you wait for him to finish his sentences will likely be the hardest part. I've heard it's better just to be patient and wait for them to finish as if nothing happened - they aren't really aware of it and bringing attention to it can have negative effects. If they do notice it, and get frustrated, then just encourage them to work through it.