Ask a Neurologist and Get an Answer ASAP
This is Dr. David. I am reviewing your question now and will be with you momentarily.
most arachnoid cysts in the brain are benign and don't cause problems with development or personality disorders or developmental delays
it sounds like your son is developing normally
an MRI scan would be needed of the brain to see more around the cyst and see if there is pressure on the normal brain tissues
but I suspect there will not be pressure to the normal brain tissues
and I suspect that intervention is not needed.
you can have his GP doctor refer him for neurological evaluation and more testing if you want to
but I suspect that the cyst can be watched safely
I am happy to review images of your child's brain CT scan to look at the cyst
you haven't even uploaded the report of the CT scan
I can't examine or interact with your son either
so there are limits to what doctors can do online to give opinions about scans we can't see and children we can't examine.