He sounds a lot like my DS who turns 7 in a few months. A neuropsych diagnosed him with Developmental Coordination Disorder. His motor skills are about the level of a 4 year old overall, but he is strangely good at swimming. The school all along has told me he is "fine" with gross motor and there are plenty of other kids who are worse. Hah! I don't think so! The school FINALLY did a comprehensive eval including gross motor, saw the problem for themselves, and they are adding adapted PE to his IEP. He will still go to gym with the regular class but will also have pull-outs to work on gross motor. Also pullouts for handwriting.
His WJ achievement score was, strangely, over the 90th percentile for writing. (fluency was lowest at 80th). Gifted kids learn to compensate in so many ways or have strengths that hide the disability. So make sure you get testing that just looks at the physical aspect, as well as other kinds of testing so you can see any discrepancies and figure out if this is purely a motor issue, or if there are other learning disabilities. There are a few different kinds of dysgraphia, for instance. DS has motor dysgraphia. He is very strong with spelling.