I used to think this was worse than it is, but this seems fairly common.
I agree with the others that his hearing needs to be thoroughly checked, and I'd go to a speech path at this point for an assessment.
The fact that everything else is on track is a really good sign.
Yes I was thinking is he hearing okay ,,maybe ask for a test to make sure all is well there .
Well, the doctor does not have to live with a child who cannot communicate with his family. My firs thought is to have his hearing checked. If he is not hearing his world clearly, he will have a hard time learning how to say words. He may have fluid in his ears or something fixable that will help him. I would do that first. Had he had ear infections or reflux??