I could break it down to him in a different way like $250 is for rent and $150 is for car insurance and phone or whatever. Not sure this breakdown is necessary.
I would break it down-we do it this way for our dd. we could just bundle the amount her bank account auto 'pays' into ours each month for cell phone (her share-less expensive to be on our family plan), auto insurance (her share-less expensive to be on our policy) and renter's insurance (bundled w/our policies which I get a discount for her on if paid in full yearly vs. monthly installments w/ a 'convenience fee') but I want her to see each month on her statement/each time she goes on-line what each of those expenses amount to every month.
I have to say though-I'm on the same page with those who feel the
'rent' is WAY TOO LOW. in reality what you are calling rent is his phone, insurance, housing, utilities, internet (albeit unavailable to him now), likely cable or satellite tv, food/toiletries/laundry & dishwashing supplies. NO WAY is he ever going to grasp the actual cost of those items and how he has to tailor his spending habits to cover these UNLESS he is paying the fair market value.
dd had the option of living at home-room and board free while going to college. one year into it she decided to move out. now, she gets astronomically low rent ($250) by virtue of a hoard of roommates but it's been a HUGE wakeup call/adjustment in her spending habits by virtue of having to shell out her share of utilities/internet/tv, buy her own toiletries/laundry/dish stuff, BUT ESPECIALY FOOD. she initially thought she could just continue on with her eating out ("I never eat that much at home anyway

) but she didn't think about the snacks she grabbed, the condiments she used, how many ingredients it took to make what she perceived as 'simple/cheap food' (she was floored to find out what it took spice wise to make a few simple items-even more so when she went to buy the spices and found out that you couldn't just get the couple of tablespoons of each but had to buy full bottles which aint cheap).
what used to be the 'entertainment' portion of her budget has drastically shrunk-no more monthly trips to gamestop (yup-she's a gamer too), daily eating out with friends and weekly trips to the movies. she's learned what it truly costs $$$ wise to self-support.
you're doing these spreadsheets with your son-try doing one for yourself. think about EVERYTHING your son is provided by virtue of living at home-
food (including condiments, spices you use to prepare stuff, foil for preparation.....all the elements of food preparation)
beverages
paper products
laundry supplies
dishwashing supplies
power
water
heat (whatever source)
oddz and endz (is there always a supply of windshield washer fluid he can take from, oil for his car, car cleaning products you keep on hand?)
this stuff you can somewhat divide by the number of people in your home and then figure out what his true cost of living for these items are.
BUT WAIT-you need to add in,
his personal toiletries (weather you buy them or he does)
what internet, tv service would cost him ON HIS OWN (because on his own he would be paying for it entirely).
try adding all of this together and I suspect you will find that you are grossly undercharging him b/c just in the realm of food-
according to the usda-for a male in their 20's the average cost per month for food (as in groceries-not eating out) is $303.60 (this is for a 'moderate' style of eating-not bare bones ($187.20) nor 'liberal' ($373.90).
at the current amount you are charging-he's not even coming close to covering his food let alone any form of 'rent'.