The FTHB credit is subject to all offsets, so yes, if he owes child support it will go there. if his wife files an injured spouse form, she could get half of it.
Since you bring up rent-to-own, there's no guarantee that they qualify anyway. The IRS will consider it if they can show equitable ownership and that will depend on the contract they signed. Rent to own contracts generally won't qualify as they are actually renting with a small portion of the rent going to escrow.
Q. Can a taxpayer claim the first-time homebuyer credit if the purchase is pursuant to a seller financing arrangement (for example, a contract for deed, installment land sale contract, or long-term land contract), and the seller retains legal title to secure the taxpayer's payment obligations?
A. If the taxpayer obtains the "benefits and burdens" of ownership of a residence in a seller financing arrangement, then the taxpayer can claim the credit even though the seller retains legal title. Factors that indicate that a taxpayer has the benefits and burdens of ownership include: 1. the right of possession, 2. the right to obtain legal title upon full payment of the purchase price, 3. the right to construct improvements, 4. the obligation to pay property taxes, 5. the risk of loss, 6. the responsibility to insure the property and 7. the duty to maintain the property. (New 7/2/09)