In Indiana, do I have to pay child support if my son is 23 but still enrolled in college?

Depends on the terms of your support order.

In general child support in Indiana stops at 21 so long as they applied for it to continue due to ongoing acredited education. Your order for support may be different and for that you will have to read it. Also if arrears are owed then those will need to be paid off. But your current obligation may or may not be over depending on how it was written.