I'd look at the whole kid, not just the academic skills. How is his language now? Social skills? Self-management (i.e. sitting at circle time, following the line leader)?
If there is any 2e, including fine motor, that will affect testing. Most achievement tests include writing; you might want to look at the particular tests and see if he scored lower on the writing-based subtests.
If you really want IQ numbers (and I don't know that you need them if he's already being accommodated well at school) there are alternative tests that some quirky kids do better on. The Differential Abilities Scale is one.
It is a great blessing when a school just notices needs and does the right thing.
