Is this possible? Should my nephews mom be able to carry him on income tax?

Since your nephew has lived with you for only 5 months, you would not be able to claim him as a dependent on your tax return.

In order to claim a dependent on your tax return, the person must be your qualifying child or qualifying relative. Please see the requirements listed below.

In order to be a qualifying child, the person must: 

• Be your son, daughter, stepchild, foster child, brother, sister, half-brother, half-sister, stepbrother, stepsister, or a descendant of any of them
• Be under age 19 or under age 24 and a student at the end of the year OR permanently and totally disabled
• Have lived with you for more than half of the year
• Not have provided more than half of his or her own support
• Not be filing a joint return for the year

In order to be a qualifying relative, the person:

• Can't be your qualifying child or the qualifying child of any other taxpayer
• Must live with you all year as a member of your household or must be related to
• Must have gross income of less than $4,200 for the year
• You must provide more than half of the person's total support

For more information on claiming a dependent, please see this TurboTax Help Article or IRS Publication 501.