Just my two cents: while pushing a kid too hard can do damage, not pushing them enough for what they can do (especially if they want a challenge) isn't exactly harmless, however well-intended. I don't know how much help I could be at determining where to strike that balance in a real-world setting, but unfortunately, for a gifted child in a curriculum where they face little to no challenge, the old adage of "just try to do your best" leaves them without the experience of learning to try very much at all.
If the school proves absolutely impossible to negotiate with (it does sound like the principal wants to avoid taking risks here), and homeschooling, private schools, etc. are not options (private schooling certainly was never an option for me as a child), just the opportunity for your son to select advanced books from a library or bookstore to read, books that are advanced enough to be a challenge yet not so advanced to put the kid off, and to practice some skills that haven't yet made it through the curriculum, even if they aren't through a formal program (like when I sprained my arm in second grade and my dad taught me how to add and multiply fractions, which was before we even did our multiplication facts in school).
Also, the way I made it through the boredom of school, particularly in kindergarten and first grade where there was so little to occupy us, was by tuning out and thinking intensely about problems - whether it was thinking about the best way to dig to the bottom of the sandbox with minimal collapses, how to count the sum of all the numbers from one to one hundred without doing the grunt calculator work, the benefits and drawbacks of capitalism, the flaws and positives of the school system, what does it really mean to think and be aware of thinking, can you figure out the EXACT ratio between the circumference and the diameter of a circle (I had figured out it was approximately but not exactly three, but I didn't know it was called pi yet), or how languages evolve.
Even if the school situation doesn't improve, or improves minimally (by instituting a GT pull-out program that fails to address the rest of the educational environment, for example), one of the best things to do is just to have involving discussions on topics that interest him, so that even if his motivation at school wanes further (which obviously no one wants) at least he doesn't lose the passion for learning new, challenging things, and sees where the problem lies, instead of branding yourself as lazy and manipulative from the get-go and living up to that for years (especially easy when, as in my case I was slower to learn to write on the dotted lines than the kindergarteners and the lunch supervisors shouted into my ear, after I was briefly unconscious in a seizure, "Don't play games, we know you're not stupid!" since they thought I kept ignoring them on purpose and making fun of me). Whether it's about dinosaurs or politics or books or astronomy or history etc., it's good to keep the intellectual juices flowing, not fixing the dam, but at least keeping things from falling apart.
