I think, if he is curious and asking persistently, he will be unchallenged during that part of school anyway. I would be concerned that working hard to put off his curiosity would just be frustrating and make him less excited about learning and he'd still be bored. I don't know that you have to lie, exactly, to the first grade teacher, but maybe planning more of a gentle, "Oh, he just picked it up, I don't know where," would be handy. Kind of like, IME, talking to people about extended nursing or cosleeping, I know there are times where I simply volunteer less information.
Also think it's a terrific idea to let him go deeper in other content areas like drawing, history, science, rather than math or reading basics. But if he's begging to know more phonics, he's accelerating anyway.
that one is a big concern of mine.
He has a trial day at the Catholic elementary on Mar 1st. i shall have a better idea on what a first grade teacher might expect after that (I hope!). Problem is, requesting early entry, particularly for a boy, is going to light up that "one of those moms" radar.