If he was already assessed by a dev. opt. I think you can rule out vision issues as a cause of the low coding score. It could be a combination of things, like perfectionism with the test and wanting to be very accurate in copying the designs, possible anxiety during testing, slow processing speed, somewhat poor fine motor ability, etc. If you get fine motor testing and that turns out Ok, then I don't think you need to worry about it.
DD's coding score rose dramatically in a period of a year. Normally you can't repeat the WISC within two years so the second administration wasn't really a valid test, and the psychologist who gave it to her was overall incompetent, but I suspect that coding is a score that is not all that reliable.
