Is there more to speech delay?

My son is nearly 2 and a half and is not speaking yet. He has been seeing a pediatrician since birth. His pediatrician thinks it’s an isolated speech delay and referred us for a hearing test (which was fine) and to speech therapy which he has been attending for 6 months with very little improvement. We have recently started PECS and he seems to be picking that up quickly and is less frustrated as he is beginning to be able to tell us what he wants. Our gp suggested a second opinion from a different pediatrician and he along with most professionals who have seen him think that my son has some autistic traits, but not enough for a diagnosis. We are about to have an ot assessment as he has some sensory seeking behaviours and over-responsivity to sound. My question is, is it possible there is something wrong with his throat/voice box/larynx etc? Should I take him to an ent or will they only be able to test his hearing? I feel that my son wants to speak, but something is stopping him from literally spitting it out. He can make sounds and has attempted probably around 15 words at different times, but not consistently. We've been told we are doing everything we can with speech and ot, but he is frustrated and so are we. I feel even if he has autism, we are missing something else! And my gut tells me it's something physical.

[jdtm]

Have you considered developmental apraxia of speech?  Just a suggestion....

[teddybears4ever2]

That was the first thought that I had jdtm.  

[MummaD]

Thank you, I have not considered this but will do some reading on it. Thanks again for your posts - much appreciated!