K.C.
The main thing is to NOT draw attention to it. You don't want him to start feeling selfconcious about it, that could lead him to be fearful of talking. When it happens, have him stop, take a deep breath, and try to have him see if there is another way to say what he wants to say. If he has trouble with particular sounds or letters, try to find other words that he can use, and work on the trouble letters independently.
The most important thing is to never let him feel like there is something wrong with him. Good luck!!