From what I've seen, lots of gifted kids have average processing speed. That doesn't mean they're not gifted, it means that processing speed is not the thing that makes them gifted.

The coding subtest requires some visual/motor ability which is not strongly related to giftedness.

If you are not seeing any signs that he has unusual difficulty with writing, you probably don't need to be concerned. Did the psychologist raise any concerns over the lower (relatively speaking!) PSI?