I'm not sure what symbol search even is (sorry). Has he had an eye exam? Maybe try a developmental optometrist. Does he have problems tracking when reading or lose his place? Also ask him if when he's reading, the print overlaps or looks double, or is moving around. You could try putting a colored overlay on top and see if that helps. DS's OT did a visual perception test, called the TVPS, you could ask about that. It has 6 or 8 subtests to identify specific visual problems.