I'm single and no child. My 401K beneficiary is my nephew. Should I pass away and he "inherits" my 401K, what will happen to it? How much will he be taxed for it? Is there a way to avoid any penalty or taxes on it? How do I go about this? Thanks.
He will receive it just like any other non-spouse beneficiary you could have named.
The money can stay in your 401K account if he wants to keep it there. For simplicity, your nephew will want to roll the money over to another qualified account, such as an IRA. The account must be titled in your name, for the benefit of him as beneficiary (aka, "beneficiary IRA").
Regardless of where the money is kept, he will be required to start withdrawing the funds a little bit each year, known as the Required Minimum Distribution (RMD) and it will appear as taxable income to him each year. There is no early withdrawal penalty in this case. Optionally, he can stretch out his RMDs over his own life expectancy. He would do this to lower his potential tax obligation, and to keep the money in his account longer, hopefully growing over time.