If it is an arrearage of past due child support he owed you, you are absolutely entitled to that money. It is not "for you" it is for both of your son. Did he not cost anything to raise during the years your ex-husband did not pay child support? I would itemize the costs of raising your son from the time the two of you split to now, from diapers, daycare, doctor's visits, insurance, food, etc. Put it down on black and white. Who got off cheaper? I'll bet dollars to doughnuts, he did. It is very expensive and hard to raise a child. And just wait, the teenage years are even more expensive.
I would not argue with him about it any longer. Show him the itemized expenses, and tell him you are not going to discuss it with him any longer. Also, don't drag your son into this argument either. It is not something he needs to know about. (Big people argument.)