We have similar stories, except that my son will be turning 7 years old next month. Since birth my son was "normal". Over the years he has hit all of his milestones rights on time, except speech. He's slightly above-average academically, but severely below average on speech. We have NEVER been able to find an answer as to why. It has been frustrating at times.
I urge you to contact early on as soon as possible and get your son enrolled in that, or something similar. My son began with early on through Haslett Public Schools when he was 2 1/2 years old. You can use a private speech therapist, but they are very expensive and typically not covered by health insurance. I have Blue Cross Blue Shield and they wouldn't cover it. While my son is enrolled in regular school now, he still attends speech therapy with the school's speech pathologist.
Please contact me if you have more questions, or if I can help in any way. I know what you're going through and know that over the years I wish I had other parents to talk to about what we were dealing with. Good luck!!