From your description, it also sounds like your child did not receive a complete IQ test, including performance as well as verbal factors. It is quite possible for a person (of any age) to be gifted in one learning area (or subject) and not in others.
In your child's case, however, we can at least assume he has significant verbal learning potential. Encourage this with lots of word play, including books, play-writing, rhyming games, etc. Also encourage drawing, practice with scissors, gluing words and pictures he cuts out on paper, and so on. It's not uncommon for young gifted kids to avoid the fine-motor writing skills because they are better at talking, and you want to encourage the manual skills needed for writing and drawing. For performance -- or the "doing" side of learning as I call it -- puzzles and building toys are excellent.
As for reliability of scores, fully retest your son when he's five years old and on his way to kindergarten.