529(b) and room/board reimbursements

This may look like a repeat question, but I haven't been able to find a definitive answer (short of calling the IRS...)

 

My son is living with us (his parents) while attending college. Can I reimburse myself from his 529(b) for room/board/living expenses? What are the exact mechanics? Do I simply withdraw the exact amount the school gives as the allowance, e.g.:

 

allowance.jpg

Source: https://finaid.umich.edu/getting-started/estimating-costs

 

Or do I need to save every single grocery receipt and utility bill and then figure out what his percentage is? Or do we need to have an actual rental agreement? (doesn't make sense 'cause it's my own money that he'd be paying me with, plus rental income tax comes into picture...)

 

Thanks!