Do I need receipts to use 529 funds for food for my college student son living off campus? Can I pay mortgage with 529 funds?

My college student son lives off-campus and is not enrolled in a college meal plan. I know I can use 529 funds to pay for his food.  The problem is it is too big a pain to save receipts for every grocery or restaurant purchase.  Am I allowed to estimate his food expenses based on the the posted yearly off-campus Living and Indirect Expense Allowances for his college, which is  $18,144.00? I think it is reasonable that  1/3 to 1/2 of this amount (about $8K) can be considered food (versus housing.)  Can I just withdraw $8k from the 529 for this?  Or is this risky without receipts?  I suppose I could use credit card purchases categorized as food or groceries, but he buys a lot with cash.

 

As for housing costs, he lives in an off-campus house I purchased as an investment.  His housemates pay me rent.  This was cheaper than living in the dorm.  I think it is too complicated to try to use 529 funds to pay for utilities or mortgage (especially since I bought the house with cash from a HELOC on another property) but let me know if anyone thinks otherwise. 

 

Thanks.