P.,
Is your son in pre-k? They should have a speech teacher there that can pick up on these things very early. If not, I would call my Pediatrician and ask for a name for a child s speech therapist.
My reason for not waiting is that I have dealt with two situations concerning speech (both boys). The first boy you couldn't understand his words. He was 3 and at the age of 5 he finally got better. But, I got his parents to take him in and get him evaluated at 3, and he did need intervention, and now he is doing great! He's about 9yrs. old.
Secondly, and I'm positive it's not your sons problem, is that my friend has a son that would not speak a word! The doctors all told my friend, don't worry, he's a boy, they take longer. Well, at 3 years,I finally talked her into taking him to a specialist. He ended up being severely autistic. Still not speaking at the age of 6. But, he has had some awesome early intervention and is learning other ways to communicate.
I myself would ease my mind and take him to a speech therapist...
Best of luck to you..............
Rhond