If he is not in a public school yet, contact the speech teacher in the school that your son is going to attend at school age. Also, please check if your insurance covers speech therapy. If yes, ask your pediatritian refer a speech pathologist.
My son started speech therapy at the age of 3 because of articulation impairment. We received the free service from the public school district even though he was not enrolled in the system as a regular student.After 2 years of therapy (once a week for 25-30 minuetes), he was dismissed last December (5 and a half). Although I think some articulation problems will self resolve as the kid grows older, I also believe that early intervention is very important.
Good Luck.