“When we initially filed for child support, we agreed that we would drop the back child support and add on to the minimum monthly payments instead. Now he is wanting to back out of the “add-on” and claim that the add-on was for a different reason that is no longer an issue. Can I go back and file for the back child support that was never paid, even though it was agreed to be dropped at the time?”
This will be governed almost entirely by the terms of your actual court order dealing with that agreement. If your court order says that back support is expunged and no longer owed, then it would probably be very hard for you to file for it again. Similarly, the order should state what the add-on was for. Generally speaking you are both bound by whatever is in the order.