You might be interested in Caroline Bowen’s wonderful web site. She is a speech/language pathologist and has a section about ages and stages of speech intelligibility. Check it out at http://members.tripod.com/Caroline_Bowen/home.html.
I would seek out a speech and language evaluation for your son. Start at the local public school. A three-year-old should be eligible for evaluation, and I urge you to get one. Beyond any general information you read, you need to have your concerns answered by a professional who is acquainted with your son.
Another source would be a local physician: ask for a referral to a speech/language pathologist. Also, you can call the American Speech-Language-Hearing Association (ASHA) for the names of qualified clinicians in your area. Visit www.ASHA.org or phone the ASHA Action Center at 800-498-2071.
Ruth Alice Jurey, M.S., C.C.C.