Why doesn't he just change it? You can change it at any time. Or do you not want to give the government an interest free loan for a year?
I think ladyknight is saying that they usually owe
money at the end of the year. I try to keep things so that I come close to even--either I owe a few hundred or they owe me a couple hundred.
There are many things that can be used to either reduce your taxable income ("deductions") or reduce the amount of tax you owe "credits")
You can either use a specified flat amount for deductions ("standard deductions") or list everything you can find ("itemized deductions.") Younger people whose income is primarily a salary and who do not have children or own a house usually find that th standard deduction is better for them. People with complex finances usually itemize.
Examples of things you can deduct:
Contributions to a legally-defined charity: money, things you donate (e.g clothing), and even your cost of transportation to be a volunteer.
Business expenses not reimbursed by an employer. This can be travel, a home office, buying a uniform, computer equipment, your accountant's fees, paints (if you are an artist), cameras (if you are a photographer), guitar strings (if you are a musician), maintenance of your rental property, and many other things. This is very complicated and most people would need help.
Investment or gambling losses--but only up to the amount you made on the stock market or at the casino. So if I lost $1000 on the slot machines and won $500 at poker, I can deduct $500 of losses. This is also complicated, especially for stocks and other investments.
Medical expenses, if they total more than a certain percentage of your income. This could include remodeling your house to accommodate a new disability, or feeding your service dog, as well as doctor fees, medicine, etc.
Contributions to certain retirement investments.
Interest on your mortgage.
Real estate taxes
There are many more. That's why it can get so complicatd.