Several things to keep in mind if you go this route. First, no matter what you do this *WILL* raise flags with the IRS. No question about it. When that happens is anyone's guess. But typically it occurs 24-36 months after the tax return has been filed and processed.
Now if you the parent are the named beneficiary on the 529 plan, I have no doubt the flag will be raised at the IRS before your tax return processing will be completed.
If the student (your son) is the named beneficiary, then you need to have a formal legal rental contract between you as the landlord (not as the parent) and your son as the student (not as your dependent). Furthermore, you need to report that rental income on SCH E as a part of your personal 1040 tax return. Take special note that while you can charge anything you want for the rent, the amount of rent claimed against the 529 funds can NOT exceed what it would cost the student to live on campus, unless the student has a formal letter from the school informing them that campus housing is not available. Such a letter would be required for each semester.
In order for rent to be claimed against 529 funds, the rent paid must be "in direct support" of the education. (this is what will raise flags with the IRS if the student is living at home.) Now while I can't be specific, I do know that the IRS has a problem with a parent charging their own child rent to live at home in what is basically their own room they lived in the first 18 years or so of their life without paying rent. They typically view it as attempting to defraud the IRS. So you and the student can fully expect to be audited at some point in the future. From all the stories I've heard "through the grapevine" on this, you will most likely lose.
So if your student does not enroll and attend as a full time student for the summer semester, any rent paid is *NOT* "in direct support" of the education. Therefore it would not be an allowed expense against 529 funds.
As for the "board" part of room and board, I would recommend you don't even go there. Just include the cost of food as part of the rent. But keep in mind that whatever you spend on food for your own child is just flat out not a tax deduction by any stretch of the imagination.
Now me personally, I would not do this. I see it as hanging out a sign that reads, "Hey! IRS! Audit me now! Please! Hurry! Quick! Fast! I want to pay lots of back taxes, interest fines and penalties!"
