M.R.
Ask his teachers -- not only his main teacher but if he has teachers for things that excite him, like gym class or music -- if they also notice this, and if they perceive his speech as being different from what they hear all day, every day. Teachers can be really good at knowing if a child is typical or not compared to peers at the same age and stage. If his teachers aren't concerned or say yes, this is typical for a child this age, then don't worry about it. If they say it's noticeable that he is sometimes unintelligible, then ask them if an evaluation by a speech therapist would be a good idea.
This might be a case where the issue is not a speech problem at all but just an issue of his being excited, and the solution may not be speech therapy but instead, working with him on controlling his excitement enough to make himself understood by slowing down.
Overall, it is typical for some children, even up to ages older than six, to speed up their speech, stumble over words etc. if they are very excited. They key here is that it's happening in circumstances when he's just so excited by something, or by the story he's getting to tell, that he can't get the words out fast enough - he's got so much in his brain he wants to tell you. If you're still concerned, get him evaluated by a therapist but first I'd talk with the teachers who see him and other kids his age every day. It's worth noting that, as you put it, he's "perfectly intelligible in most situations."