L.,
Ordinarily, when someone askes a question about delayed development, my antena goes up and I give the same advice, to go to a professional and find out four sure, because if you think that there is a problem, there usually is. I don't really get that impression from your post though, because you only mention the one area of concern, and while kids with asperger syndrome, as some have suggested, do have gross motor difficulty and can have high IQ's, most parents will be concerned about fine motor, behavior, and social skills before they put two and two together about the generally less obvious gross motor or coordination issues. The only thing in your post that makes me wonder that you note his inablity to understand the rules of an organized sport, if you think that this is a language issue, then you might have cause for concern.
I would say, seek a professional opinion if you have ever wondered about the following: His language usage being little professor like, or just something a little off, or the tendancy to monologe about one topic which he knows many details about. If he has an odd quality to his voice, either too much or too little expression, too loud or too soft, monotone, or irregular usage. Trouble relating to his peers- if he is markedly better at getting along with younger kids or has a distinct preference for adult company and does not seem to know how to make or keep friends his own age (even if this seems to be just a prefrerence.) If he has trouble holding a pencil correctly, or has trouble with buttons and hygene tasks independently. If he has trouble with any sensory experience or becomes agrivated or overwhelmed by noise, touch, light, or taste. If he has difficulty with attention span or impulsive behavior. If he seems to have too much or too little fear of strangers and either fears or has anxiety about novel situations, or thinks he knows more about novel sitiuations that he really does. There are more things that could cause you concern, but most of them will have something to do with these kinds of issues and could present in as many different ways as there are kids.
Lastly, you are seeing one thing that is universally typical (normal is not really the word you are looking for) and that is that kids can and do excel in one area and not others so if you really "can't" understand that, you should find a way to understand it as soon as you can. Acceleration in only one area does not mean anything is wrong with the others, and you should certainly not become concerned about his skills as compared to one child who is only one year younger any more that someone should judge thier child's reading ability as compared to your son's (the "typical" range is huge, so what you see as "significant" may not be so in a clinical sense.) Another caution to you is that he may be doing very well right now, but his acceleration does not always mean that he will continue on that path and you should not assume "giftedness" until the end of third grade as many kids who learn at an accelerated rate that will even out by the end of third grade. Even if he is advanced in reading, he may not be later in his life, and you should evaluate if you are putting too much empasis on him being "advanced," and let him be a typical kid instead as he could be holding back in areas that he perceives you will find disapointing by comparison.
So, unless you left a whole lot out, or you have never thought of some of the areas I listed as problematic before, then I would just relax and give him a chance to work on his gross motor development.
M.