GMAT used to offer a paper based test where it is easier to track, as an accommodation. I don't think it does so anymore but it's worthwhile checking, since GMAT changes things now and then and they make special exceptions. If it does, get a psychoeducational evaluaton and see if there is a rational...

I don't purport to make clinical diagnoses over the internet or email, but his comments at face value sounded like ADHD and I recommended further evaluation with a neurologist and a neuropsychologist, with the add on that there may be other (or different) issues involved like a reading a disability ...