C.M.
You definitely can ask for it to be retroactive. Usually what the court does is increase his monthly amount by a nominal amount to go towards the arrears. My ex has to pay an additional $50 a month towards the arrears. But, you do have to have that in the child support documentation so I think you would have to have amended papers, which will probably require postponing the hearing tomorrow. Your attorney will tell you if that is the case or if you can just request it in court tomorrow.
My opinion only, but I would absolutely ask for the arrears. Like you said it is money owed to your son and he most definitely deserves it!
Best of luck tomorrow and going forward!