Legally, no, he's not.
The state in question is CA, if you would have read the first post. You can't "ask" for arrears, the court orders them. CS will go back to the date of filing. No further.Your ex can ask for arrears. It depends on what state you are for how many years back they can go.
Actually, I wouldn't trust you as far as this statement is concerned. You cannot go immediately after a modification after an order was just written.

