Yes, this is a simple request for modification and can be handled by your state's child support agency. You should be able to find your state's child support financial worksheets and guidelines on-line. You can use them to figure out whether or not going back will help of hurt your situation. For example, if you are also making more money than you were when the original order was put into place, you could end up paying even less. It doesn't sound likely, but that's what would happen if I went back and requested a modification on my order. It was set when my son's birth father refused to submit financial records so they assumed he was unemployed and then, using my actual financial records, adjusted his support DOWN from the standard set for someone making minimum wage because I had a real job and made more than minimum wage. Later when he did get a job he still made less than me and my income had increased more than his did so his order would have been reduced from $45 a week to something like $32. Not that my order has ever been enforced anyway...
So check the formulas first to make sure you're not shooting yourself in the foot and if you should get more $$, call your child support enforcement agency and request that they do a modification for you. If they refuse (not enough time has passed, the income change is not "substantial" enough) you can request this yourself without an attorney. If you check your state's regulations on-line you can find out what constitutes a "substantial" change - I think in my state it's a 25% difference in the support order - and make sure that his income change meets the criteria before filing. Good luck!