You need to look at words of your order---generally an order spells out a specific $ amount, not a specific % of income.
And the amount he owes does not go up or down --even if he hits the lottery, until and unless the order gets changed. And generally a change does not apply backwards.
So if his counted income went down it may NOT be to your advantage to wake him up to seek a lowered order but to allow the arrearages to accumulate for a while longer before you press for payment of same.
Obviously it doesn't make sense to allow arrearages to mount up so tremendously large that its impossible for him to pay, you cannot get blood out of a turnip.
I think you need to lay it out on paper more carefully and discuss it with counsel familiar with quirks of IL law.
BTW, IL law leans hard on view that he needs to help pay for college as well---if he opens up CS can of worms, make sure college is added back in if not already there.