For 2017-18 college year, our 2015 income was used to apply for FA. In 2015 we had to exercise our company employee stock due to the deadline, the proceeds which ballooned our income for that year. While the FA offers were wildly varied among those schools that accepted my son, he had already planned on taking a gap year irregardless, so the atypical income in 2015 became a moot point. When we file for FA again for 2018-19, it’d be based on 2016 income which is our typical. Although VERY unlikely, do you have a gap year in mind? 
I’m not sure how much that stock represents both in absolute $$ amounts and in terms of your overall net worth, but the fact that you mention stock, singular, poses an even greater risk than being in diversified equities overall. You started this thread in March with your oldest in 10th so now your child is in 11 and your time horizon for college is less than two years away. Normally, with a short time horizon, one should be seeking less risky investments. If you don’t need the $$ in 2 years , that’s a different story however I’d still think you should be more diversified.
How would you respond to losing, say, 20-25% of the security’s value overnight? The S&P 500 has been up 21% over the past 12 months so it could easily correct by a similar amount.
Is it a single stock or a stock portfolio?
There’s an old Wall Street saying: Bulls make money, Bears make money, Pigs get slaughtered.
You want to make sure you aren’t being that pig by hanging on to risk you can’t afford in a desire to make more $$. Very few people are good at timing their exits at the tippy top.