If ex increases child support willing to avoid going to court, 7 years later can he stop paying & to say he has a credit?

The case will hinge on whether the Court accepts that you both agreed that the increase was to apply to any prospective balance. Any agreement made outside of the court is enforceable only to the extent the Court is willing to acknowledge the intent of the parties and what impact the agreement has on the best interests of the child(ren) involved.

What should have happened, is that you should have gotten the voluntary increase in the form of a Consent Order Modifying Child Support, so that he then would have been obligated to pay the $1,200/mo. As it stands now, it is possible for the court to apply his overpayment to his balance. Not sure what you mean by "to avoid going to court". If there was a Motion to Modify filed and pending at the time he voluntarily agreed to increase the amount, you could now have the court calculate what the amount should have been from the date the motion was filed. If he was supposed to have an increase done at that time, he may not get quite as much of a credit for prepayment (in fact, its possible he could have an arrearage amount that he is behind on). If you don't have a Motion to Modify pending, you may need to file one. Consult with an attorney familiar with Child Support, to get more specific advice, depending on several factors in your case.

If the prior order wasn't modified by the Court he has a goid argument for an overpayment credit. You should have consulted an attorney then to protect yourself. Court orders cannot be modified outside of court. You still may be entitled to an increase as the original order is more than 3 years old depending on the parties current incomes. Hire an atty to assist you. In the long run they may save you lots of money.