3 yr old language development

My 3 yr old has a pretty good size vocabulary however he does not say the first letter or sound of the following letter: C, CH, G, J, K, S, T, V and Z.  This makes communication between unfamiliar people difficult as they can not understand him.  Oddly when we "play" sound repeating his is able to make the sounds, but when they are at the beginning (only) of a known word, he omits it?  Is this something other people have encountered?  What did you do to correct this or did it resolve itself.  Half of me says he is just three, and since it only affects the sound at the beginning of a word, I am not sure how to proceed.  Thanks

[Hazel30]

My son too was having these issues since he was 2.  I took him to 3 speech classes and say that they do nothing.  So I took him out and since its been improving.  I think it just takes time.  He is in preschool now and it gets better, but I also notice that some kids start off right away talking more mature then others, like baby talk almost.  But I think you should focus weather you see improvement rather then how he's talking at the moment.

[Hazel30]

Just wanted to add that my son is almost 3 now and improving greatly.