School evaluations are by law just to connect what issues a child has with their success or challenges at school. Even if you have an excellent staff person who can give you a great evaluation, that is the mandate.
It is really really important to have this done in the medical or private arena to get him the appropriate diagnosis.
Schools also by law can not diagnose. For example, I am a speech-language pathologist....in a medical setting I can use the dsm and give a diagnosis code in my area of professional training, the md then making that a medical diagnosis based on my evaluation.
In the schools, I can do the very same testing but can only say what the results mean. I have to relate that to how it impacts the child in the educational setting. That is the way special education is created and funded.
Many times the two overlap....and many times the schools simply do not do that complete of an evaluation, because they only have to do what they need to qualify or not qualify a student for special education.
In the school setting, Occupational Therapist (OT) is specifically a "related service" that means that it is only used in the schools if it is required to help accomplish a goal on the IEP. Speech and Language services can be either a related service or a stand alone disability category on the IEP. There is no Occupational Therapist (OT) category in special education.
in my humble opinion start with the medical evaluations. That is where you get the big picture evaluations. They look at cognitive development, learning, milestones, behaviors, health history and birth history and social history and emotional history and issues, etc....they look at communication skills in general and motor skills in general. That is why it is also helpful to get a private and separate speech language evaluation and an occupational therapy evaluation.
Also, because the goal of school therapies are pretty legally restricted, it is often helpful to have them work toward school success and to also have private Occupational Therapist (OT) and speech/language therapy to address more global skills that help in all areas of life.
I often collaborated with private area therapists so that we knew what each other was doing and then we supported and didn't contradict each other in our therapy work.
I know it is overwhelming. The school evaluations are free. But you are likely not to get the results you are looking for in this case. They especially will not be able to help with a differential diagnosis regarding FASD or other kinds of drug exposure conditions, genetic conditions, mental health conditions, etc. That is only going to happen with the "comprehensive evaluation" which means it covers many many areas of the child's life and skills....it will include checklist and forms for the parents to fill out as well as direct tests with the child....most are pictures and manipulatives that the child does in a sort of game like format.
Bringing those results to the school will help you revise the IEP and get more appropriate services.