http://www.irs.gov/publications/p915/ar02.html#d0e341<snip> "To find out whether any of your benefits may be taxable, compare the base amount (explained later) for your filing status with the total of:
One-half of your benefits, plus
All your other income, including tax-exempt interest.
When making this comparison, do not reduce your other income by any exclusions for:
Interest from qualified U.S. savings bonds,
Employer-provided adoption benefits,
Foreign earned income or foreign housing, or
Income earned by bona fide residents of American Samoa or Puerto Rico."<snip>
Essentially, if the total of one-half of his SS Benefits PLUS all other income is more than $34,000 then his SS benefits become taxable. If your parents file jointly the figure is $44,000
I am unsure what you meant by this statement;
"I am doing a part time job which my Dad signed the W-9 for"
Does that mean you are employing him? Or is he employing you?
The bottom line is simply this....go to
http://www.irs.gov/pub/irs-pdf/i1040a.pdf and head to the tax tables section. You would need to figure their AGI and simply look at how much of a difference $2500 makes.
If your dad files singly, and his AGI would be $34,000 the difference would be about $600. ($5064 vs $5689 in taxes)
If your parents file jointly the difference would be about $380 ($4349 vs. $4729)
The above was found on that link i gave you - 2006 1040 instructions, comparing an AGI of $34,000 to an AGI of $36,500.
Hope that helps