We didn't have enough taxes withheld as we claimed out son as dependent.
However son has worked and made too much money (at least I think). So we will not be able to claim him as dependent.
Is there anything we can do now?
If you withheld taxes based on what you paid last year then you likely will not owe any underpayment penalties, just the extra tax. And this is most likely your situation. The IRS has a couple of different safe harbor provisions and for most tax papers withholding what your tax due was last year puts you into that safe harbor.
Now for this year, you will need to adjust your withholding as your safe harbor withholding amount is going to be higher - since you owe more taxes for last year.
You'll have to pay it. Gimmicks you can try are filing for an extension, letting you extend and pretend that you will have enough money to pay the government by then (October in the US). If you extend and pretend long enough maybe by 2018 tax time you'll have set yourself up for a tax refund from some other source and you won't be negative anymore, since you can file that year's taxes at the end of January 2018 in the US, giving you only a few months of financial distress in between time.