Well you have to reply to the reasons they've given.
I think you have a good response to the second one, both that there isn't a way for him to demonstrate his desire/interest in the school based class, but also he has surely demonstrated both his passion and his ability in the class at home (that would be where you bring in work samples, if you have not already).
The first is probably the stickiest. The school thinks they can't due to some policy, you need a copy of the policy...and to review it yourself and see if it does or does not allow for the thing you want, or perhaps allows for something similar, better, or close enough...
no 4th grade science? Is that policy? Maybe he can have his own 4th grade class, for example...
good luck!
