In no way am I saying that his artistic abilities or passions be held back, but I may be totally on the opposite side here. I mean if this was a math class, and DS didn't want to do fractions because his passion was decimals, it would be a no brainer, right? We wouldn't ask the math teacher to let DS only do that which involves decimals. We would ask possibly for more decimal work, whether be for extra credit or in a math club or at home...
To me it seems as if Art class is less about expression and more about instruction now for your DS as he is older, which is generally not the case in the lower grades. And with that instruction comes learning portions of art that are not fun for him... if he's a sculptor, he doesn't want to do a lesson on shading or whatever it may be. Now granted, the art teacher should not correct his actual work if it's within her boundaries for the lesson, but he also needs to learn that this is a class...
Does that make sense?