If nothing has been done to improve the issue causing the processing score problem then you will only see change in the IQ relative to the fluctuation of testing so young. WPPSI is designed to test children as young as 3 and isn't focused on reading and writing as those are more achievement related and bias against those for whom English is not spoken in the home.
A discrepancy as large as the one you describe between processing and the other scores is an indication of a learning disability. You may not learn more clues as to what that is until your child is further in school. It can take awhile to figure it all out. Children as bright as yours tend to find ways to compensate for those handicaps making it hard to distinguish what is really going on.
If you choose to test again, it might be helpful to work with a Neuropsychologist so that you can get more information about what the "processing issue" is likely related to. If your child is not yet 6, he may not be able to test on all the batteries that would be helpful just yet.
