Why do the courts do this?

Alright, I asked this question already, but I think I need to rephrase it. My husband has a teen son who recently moved to his mothers. They were never married, she ran away while pregnant as a teen with another guy (I say this because of a last response.) There is NO current child support order against my husband, although there is one still against the mother as she is still having her wages garnished from arrears she owes my husband. I'm sure she has started a case against my husband since their son moved with her, but as of yet, my husband has gotten no paperwork for it, meaning there is no court order for him to pay. Now, my husband asked me to call our local child support to ask them how to go about making sure that the payments he has been paying will be counted so that he does not have a large sum from arrears when it finalizes in court. I mean, the custodial parent is entitled to child support payments from the date he/she turns in the paperwork. As in if she applied this month, and it did not finalize until 6 months down the road, my husband would have to pay arrears for the 6 months until current. When asked how to ensure the money he is sending is counted, like writing a check with the memo stating Dec Child Support, we were informed it is not, that anything paid is a gift. That if we want to ensure the money is counted, to put it into some type of account, or saving and have it to pay the back payments off once finalized in court. Our income is limited and we also have 2 children of our own, so we cannot afford to keep paying her "gifts" each month, and then go right around and pay for those months all over again. It isn't trying to get out of it, it's wanting to be fair to not just his son, but our children as well. This isn't the first person or office, or even attorney that has told me this. Do you think this is fair though? Should the courts do something more for those parents who want to help support their children before it gets finalized by the judge, like count any money that is given to the custodial parent for the care of their kid? What about for those custodial parents who struggle and need the money, but the non-custodial parent is struggling too, so can't afford to pay it once, then all over again because it was never counted. Again, there is NO current order for him to pay, he currently does not owe anything he just wants to ensure he doesn't, and that he isn't treated like a dead-beat dad if he stops paying and instead puts it aside till court finishes.