It can take up to six months before the IRS will release it to the agency, then it can take up to 3 more months before they will release it to you.
I have gotten my ex's taxes the last 2 years for non-payment of child support. Each year he file at the beginning of February and I usually receive the money in the middle of June that year.
If he has already gotten a check from the IRS for taxes he filed before April 15, 2008, you won't get a hold of his tax return this year. The next chance you would have at getting it is April 2010.