If he makes the cutoff, he should move up--that's what cutoffs are about!

As for brains and abstract thinking, well, the whole point of "gifted" is that your brain is ahead of where brains normally are at your chronological age. If he's worked through all the stuff leading up to algebra, it's not a huge leap to algebra itself. Algebra is not a foreign language, and kids have already learned the idea of variables along the way, whether they called it that or not. He'll be fine. I have an 8-yr-old doing algebra, too.