We had a similar expereince. DS2 started therapy at 18 months, and I started the process of finding the underlying problems.Since he is adopted, the EI therapists all said it was due to that and learning a new language, but I knew it was more. Afer doing a brain MRI, which came back clear, the pedi neurologist suggested the child development clinic at childrens hopsital. DH took some convincing since it's a 5 hr drive from where we live, but we saw them when DS2 was 26 months.
Can I tell you how much I love them? They diagnosed him with autism and suggested blood work for Fragile X Syndrom, which he has. We go back to see this Dev Ped up there every year and she continues to be the Dr who I feel really gets his whole picture. Thankfully our local Pedi is open to working with Dev Pedi :)
Anyway, I would go back to the childrens hospital clinic now that he is older.