Yes, the ex can take him back to court. And, unfortunately, "appropriate" child support, per federal law, must be based on the parents' income and not the "needs" of the child. A receipt for daycare can be obtained only if the other parent is receiving add-on daycare costs and not obtaining any daycare services. But, you can't get the receipt in advance -- you have to observe the other parent cheating and then initiate a support modification proceeding.
As for the 50/50 custody issue, if you have that in the orders, then you should be exercising those orders scrupulously, and if you're not, then you are settingyourself up to losing that arrangement. And, if your orders don't actually provide equal parenting time, then you don't really have 50/50 custody.
Note: I'm actually "on your side." I believe that the present child support laws border on the insane. But, the law is what it is, and it's on no one's political agenda at the moment to change these laws -- so, if you don't learn to play (and pay) by the rules, then the child support train will run right over you like you weren't there at all.