I think, unfortunately, that art class is all about the teacher. Some teachers are great, and some are awful. IMO, the best thing a teacher can do is to introduce a concept/technique/medium and let the kids go at it. The worst thing a teacher can do is to come up with such rigid projects that all creativity is lost. It sounds like your son's teacher(s) may be doing the latter.
I would suggest that he talk to the teacher. For example, he could say, "This project has really inspired me! Is it okay if I work extra hard and [insert whatever semi-related project your son can think of that he actually wants to do]?" Most art teachers aren't going to say no to that kind of request. If his teacher does say no, I'm afraid you'll just have to suck it up and hope for better teachers in the future.
I had an art class like this in college, including the cheesy paint-a-picture-of-an-undersea-scene project, which my prof would not let me get out of doing despite my absolute abhorrence for it, and it was miserable. Yuck.
ETA: The rest of my many art classes were wonderful, with open-ended assignments, which I could switch around however I liked by simply discussing it with the teacher/prof.
