Meet with a local family law attorney so you know your rights. He cannot make you agree to forgive back past due child support. If he made direct payments to you, or he had possession of the child during the time he was paying support, he may be entitled to a credit against his child support arrears. His current child support is based on a percentage of his income. The percentage is based on the number of children.
Your decision is best made in consultation with Office of Attorney General and/or private counsel. You are not required to forgive arrears and support will be lowered based on Texas law, including support guideline and any reasons to rebut presumption of guideline. You should consult with family law counsel and possibly retain your own.