How to explain OT to a child?

We are planning an evaluation for our six-year-old son and expect to hear that he has sensory issues in some areas (mostly some specific fine and gross motor skills, plus low muscle tone, very limited diet, somewhat high anxiety level, and frequent accidents).

Our question is how to explain to him what we're doing and why, in a way that he can feel good about. He has a twin sister who doesn't happen to have these issues, and lately he's begun keenly any differences between them. And when he heard he would begin some OT through the school system focusing just on fine motor skills (writing and cutting with scissors), his response was to say that he must be a bad writer.

I'd be interested to hear how others have explained the need for evaluation and OT to a child who feels differences from other children so keenly.

I should add that he does well in school, has good friends though is shy in crowds, is learning an instrument and enjoying it, and that certain of his fine and gross motor skills are quite highly developed (skis, swims, builds complicated Lego creations--but then struggles with other skills).