The optometrist is just that an optometrist, he is NOT pediatrician who is trained in childhood disorders both physical and cognitive.
Very few optometrists are specifically trained in dyslexia, if they are the need to be accredited as such. They will know there are many aspects of dyslexia, that can affect a persons ability to read, write, spell etc, called dyscalculia (maths), dysgraphia (writing) and dyslexia (reading).
Its a misnomer that just because someone can read or write they are not dyslexic, many dyslexics will have a strength, in one area, and most will have found some strategies to cope.
Comorbidity of chronic conditions;
Asperger's syndrome, Autism, ADHD/ADD can most certainly be a coexist with dyslexia and other learning disabilities.
My daughter and I are both dyslexic, we were diagnosed at the same time. I initially took her to an optometrist to rule out visual problems, before taking her on to her pediatrician, the optometrist told me my daughter was not dyslexic. I nearly did not follow up with the pediatrician because of the optometrist report.
However the school had also raised concerns, and organized for my daughter to be tested by an Education Psychologist (E.P), she was diagnosed as having ADHD, ODD and Dyslexia, (she is borderline Asperger's). Which was confirmed by her pediatrician. We also saw a Irlens dyslexia clinician, who also confirmed her being dyslexic.
We used behavioural management and diet to help with the ADHD & ODD, with the dyslexia my daughter found the use of tinted lenses from Irlens helped with reading and writing etc.
Please follow the recommendations of your son's pediatrician, since he/she is more qualified than a optometrist.
Supporting a person with Asperger's syndrome should involve multidisciplinary team of experts, to ensure your son has the best possible outcomes. And vision therapy may help, but i would also tread very carefully.