Well I kinda thought it was my business since he told me to put it in my son's savings account and not spend it... So... Yeah. Since I did do that then I would expect him to extend the same level of respect.
You don't have to put the money in a savings account and neither does he. It is quite equal in that respect. Sorry to hear you are loosing maintenance but it doesn't sound like it will be too much of a problem. Do as you wish with what he will/has given you and move on.
So he's been paying x, but has now gone through the CSA and they've told him he only needs to pay a certain amount which is less than x. He's decided that he's been overpaying to you and that he wants it back?
Don't pay back anything until you get work from the CSA (though would they not just reduce his payment to you, rather than ask you for a payment back?). It's none of your business what he does with it though, like it isn't his business what he does with the payments you receive.