sounds like something is going on. I agree that he needs early developmental testing. You can go to "babies cant wait" which is resource for things like this, they provide money for expensive work ups and therapy and they also act as a resource broker connecting you with the people and organizations that can test him and give him what is needed to help catch up.
My peditrican said nothing was wrong with me and I had alot wrong with me from brain injury caused by a defective infant formula that contained lead and lacked certain essental mineral salts needed for brain development. I was later diagnosed with PDD with autistic charateristics. But just because your baby doc says nothing is wrong, doesnt mean anything.
You need to trust your intuition on this.
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All art is a kind of confession, more or less oblique. All artists, if they are to survive, are forced, at last, to tell the whole story; to vomit the anguish up.
-James Baldwin