Back pay child support?

my husband's ex says she's gonna put him on child support. this is fine with me b/c i think it would help out with all the bickering and fighting that they do over the child. my question is about back pay. his daughter is 9 years old and lives in alabama with her mom. whenever the mom says that the child needs anything, he buys it and sends it to her. we have been keeping receipts for about 3 years. he's in the military and we are now in korea. my question is will he have to pay 9 years of back pay child support or is there a limit of years on how much he has to pay?