Typically, scatter between index scores can be a flag for a learning or attention issue. It is very common for smart kids with LD to have either a low PSI, WMI or both. The issues you describe about directionality (although that suprises me given the high PRI - although it could be an attention thing, not a visual spatial thing), forgetting things etc.
I agree that a neuropsychological evaluation would help you to better understand his learning profile and possibly lead you to a diagnosis. At the very least, you would gain some important recommendations to help him going forward. Make sure you find one that specializes in children and education issues.
This is a good resource to learn about WISC and other tests
http://alpha.fdu.edu/psychology/WISCIV_Index.htm
