Not everyone on the list diagnoses, though, so you will need to ask. Here is what a comprehensive assessment entails so that you can ask the right questions. And, you may or may not need a psychologist. It is no longer necessary to have an IQ assessment to make the diagnosis of dyslexia. Oral language is considered to be the best predictor of reading and spelling. You didn't say how old your son is, but especially if he is older, you'll want to be sure the examiner has experience with his age group. I've seen too many middle and high school kids who were "missed" because the examiner did not fully understand what dyslexia looks like at that age.
A good assessment sets the groundwork for intervention and should spell out the needed recommendations for your son to begin to improve, succeed in school, and importantly, feel successful. I have a lot about intervention here.
I am assuming you know that if he has a suspected learning disability (LD) he might qualify for services under special education in the public schools. Dyslexia is a type of learning disability. I have a lot about that process here. That said, many dyslexics do not score poorly enough on standardized tests to meet the criteria for services in the public schools, despite clearly not succeeding with the curriculum. Moreover, these days the schools in Michigan use Response to Intervention (RTI) as an "intervention.” RTI, in my opinion, is not meeting the needs of many of our dyslexic/LD kids. There are three tiers to RTI with a different type of response at each tier. Typically, a referral for special education services does not occur until Tier 3, by which time a student is usually in 3rd grade and has failed for 2+ years. In theory, RTI is quite good, but in practice, it is not panning out to be what our dyslexic kids need.
Once a request for special education services is made (and you as a parent have a right to sign that referral), the school has 30 school days to do the evaluation. I have found that sometimes it is helpful to have an outside evaluation completed first and then take that to the school because many times the testing in the school has not been sensitive enough to detect the dyslexia. This is not always the case; I have worked with some really good school teams who are on it (i.e., doing good work with students with dyslexia).. I hope this is helpful. I think your first task is to determine whether you need a psychologist or not.