I think I'd just let it go. He's 7 and it really doesn't matter if he gets a zero on a spelling test. If you're sure of the reason he got a zero, explaining it to him, commiserate, congratulate him on spelling so many correctly and then use this as a learning moment for teacher expectations.

It's a bummer she's like that, but it sounds like she might be someone with whom you need to pick your battles.