Taxes

I just found out that my SO claims 9 on his tax form for work. He went to file his taxes today and it said he owed a lot of money. He got a return last year. So I asked him what changed. He said he has no clue. I looked at his paycheck stub and sure enough it has 9 for federal and 1 for state. He made almost $40k last year and only paid $1177 in federal taxes. Im so frustrated. He of course says he didn't put his exemptions that high, but his work wouldn't just do it. I always claim 0 so I never have to pay in. I really hope they have payment plans cuz we can't just fork over $2000 🤦‍♀️😔