Advice for getting help and who to see

We have a Problem with our son whom is 18 yo as well as trouble with doctors and processes for getting a diagnosis I have people lined up who want to help but he needs that all important diagnosis.

His mother has cancer and I have medical problems looming myself so we need to push things along.

His problems are he Still sucks his thumb at 18 years of age, sits rocking back and forth, lack of concentration, trouble learning in normal environments (School), easily distracted, extremely shy and reserved, does not want to make friends, physically small weak and prone to sickness and passing out when stressed, can’t cope with stress (completely shuts down or gets aggravated), he has no comprehension of what it takes to survive in life and is totally dependent on others for food money and shopping etc.

On the upside when we put him in Kip Mcgrath for extra school tuition he responded extremely well but unfortunately upon leaving Kip Mcgrath he dropped in performance and was removed to a special class and became completely withdrawn and upon leaving year 10 refused to go back to school.

When he first went to a job network member they told me that “the job network process would eat him alive”, most people can see he has a problem when meeting him and are supportive but the problem seems to be getting someone to commit to an official diagnosis.

It has taken years to get a referral but after seeing the neurologist I get the impression that she may be less than interested or helpful, I need to keep the momentum going but I am very unsure of what to do next, he has had an MRI but I doubt that it will show any physical problems. If the MRI shows no problems what is the next most likely step and what should we do next?

Thanks for amy advice