If your brother is under 24 on the last day of 2008 and was full time student in 2008 and lived with you more than 6 months in and your brother doesn't provide more than half support for himself - he is your qualified child and you may claim him as dependent and you qualify to use the head of household status.
See for reference the IRS publication 501 - http://www.irs.gov/pub/irs-pdf/p501.pdf
His parents are divorced and I see there is an exception for that. What is the exception??
Please refer to the IRS publication 504 - http://www.irs.gov/pub/irs-pdf/p504.pdf
If a child is treated as the qualifying child of the noncustodial parent under the special rule for divorced or separated parents, only the noncustodial parent can claim an exemption and the child tax credit for the child.
However, the noncustodial parent cannot claim the child for head of household filing status, the credit for child and dependent care expenses, the exclusion for dependent care benefits, and the earned income credit. Only the custodial parent (or other eligible taxpayer) can claim the child as a qualifying child for these four tax benefits.
However - if the child did not live at least six month with either parent - he is not a qualified child for his parents, but only a qualified child for you - see definitions above.
If you brother is your dependent - there are separate filing requirements for dependents - see the publication 501 for details.
generally - if his income is from work only - he is required to file if his earned income is above $5450 (fro 2008)
However if he has any taxes withheld and want to get a refund - filing tax return is the only way to get a refund.