I would agree with littlebit667 that he is delayed and needs time to catch up I would like to refer you to a link that may help you teachmetotalk.com/.../what-works-strategies-that-help-toddlers-learn-t...
I suspect he has been delayed partly because of the medication and medical issues. He may need time to catch up. I would have his hearing checked by an audiologist who works with children to see if there is an issue there. Sounds like you have been on a hard road.