The only thing I would worry about playing a bunch of yellow zones over the middle is if he gets enough time those slants will go deeper down field and eventually pass the yellow zones leaving them wide open.
I'd try jamming the WR's at the LOS and just bring my safeties down into yellows to cover the middle of the field better for those slants. Just be aware that if the outside CB's miss the jam, he's likely be wide open. So try to user one of the safeties and just watch the outside, if the jam goes through move up in the middle of the field. If it doesn't go through, better start running!
Like TNT said, there is a lot of ways to stop slants. Just need to experiment on what works best with your abilities and your defense.