I'm not providing a definitive answer here, so will post this as a comment. I say no you can't claim it as a qualified expense. Period. While room and board is a qualified expense for a 529 plan, since you claim him as a dependent and he lives in your household and not a college dorm or other off campus rental for which actual rent is paid, there are no room & board expenses to claim. If actual room and board were paid with those funds, then it would have to be claimed as taxable income by the recipient of that payment. Most likely, your ex who is receiving the payment, which you clearly identify as child support paid to your ex, and not a room and board expense paid to an entity qualified to receive it, does not pay taxes on that money, and doesn't even have to report it in most cases. You pay taxes on that money before you pay that child support.
If this was alimony, things would be different tax-wise.
