For this, either of you can claim it then. If your son claims it, then when the student does their tax return, the student "MUST" select the option for "I can be claimed on someone else's return". Most likely, 100% of it will be taxable to the student, and it will be taxed as the parent's higher tax rate too.
Do note that since you paid "out of pocket" expenses, if you work through the Education section in the order it is designed to be used in interview mode, then after you enter all the 1099-Q's, in the expenses section you will specifically be asked how much was spent on room and board. That will offset the taxability of the savings 529 plan. I'm not sure, but I "think" that will also reduce your AOC claim.
If you do NOT work through the Education section in interview mode the way it is designed, then you will NOT be asked for room and board expenses, and that will INCREASE THE TAXABILITY of the 529 savings withdrawal.
