I don't know about TN laws, but having dealt with Child support issues in Wisconsin, Illinois, and North Carolina I can tell you that as far as the court is concerned, your husbands children with you are irrelevant. The court doesn't care that he has two other children, car payment, mortgage, etc. In most states, your husband is responsible for half of that child's costs. Generally, states use a formula, awarding roughly 17-22% of the non-custodial parent's net income as child support payable to the custodial parent.
Do a Google or Yahoo search for Tennessee Child Support laws and see if you can find something that applies to your situation. The TN Child Support Enforcement office (every state has one) may be able to answer some questions - but remember that they are in the business of collecting support, not helping non-custodial parents get their payment lowered. TN might have a legal aid dept that can offer advice or maybe even legal counsel, but you'll have to look into that. Most lawyers will offer a free initial consult, it wouldn't hurt to go to a couple and see what your options are (as well as find out how much a lawyer would cost if you had to go that route).