I think it is great that the school is at least aware that sometimes handwriting difficulties indicate issues that require OT and that they don't look at it as a character defect or as laziness or lack of motivation. I don't think that an OT evaluation is a bad idea. If you have a thorough evaluation from a good OT who is familiar with gifted kids, you will either find out that your child's fine motor and handwriting skills are developmentally appropriate, or that they aren't.
If you find out that they are appropriate, you can share this information with the school, which may help you get them to back off on the pressure, if they are expecting output that is unreasonable for a normally-developing child. Advanced intellectual abilities don't necessarily imply advanced physical abilities, and age-appropriate fine motor skills should be perfectly acceptable. They can't really reasonably demand that your child needs to somehow alter the rate of his normal physical development (and I would absolutely phrase it that way if I ran into resistance.)
If you find out that there is an issue, then you are in a great position to both address it early and to get accommodations and support in place for your child before it becomes a huge source of frustration and loss of self-esteem.
