My son started stuttering when he was 3ish...but not a REAL stutter (being unable to make certain sounds, but able to make others...like catching on "k" but not "ch" = k-k-k-k-ids or k-k-k-k-k-children)...his was one that was obviously caused by his mind running faster than his mouth, or conversely, losing track of what he was saying mid word. Doh.
My mum was worried, but we weren't. Of course, I have a good friend with Tourettes, so my stutter tolerance is pretty high to begin with, and the toddler-stutter my son developed didn't seem to be related to any specific sounds, but instead to his relative level of excitement or concentration. He's mostly outgrown it, except for when he's reeeeally excited. Took about 2 years.
All that said, if you're nervous about it, speech pathologists tend to be amazing people with a ton of helpful tips and tricks. You should even be able to get him evaluated through your district for free.
Good Luck :)