You're absolutely correct. The real world isn't that simple and easy and he needs a dose of what it costs to live. You've done a great job at thinking this through, but I have a few additional questions: What are his car or other expenses? Has he learned to pay bills? Has he learned to save?
I'm going to assume that he pockets the $272 for my answer, but you may need to adjust if he is, for example, paying out car insurance. I would take up to $150 per week, with $100 being a forced savings that he knows about and $50 being room and board. The $100 can be used upon your approval for a car, apartment, etc. The $50 is up to your family's financial situation. If you need the money, by all means, use it. He is an adult and you do not have to foot the bill for all of an adult's expenses. But if you can afford to save it, you can gift him with things like furniture, grocery gift cards, etc. when he moves out.
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Answers from the Parenting of Adolescents Forum:
From JCMOM1: "I am actually just going thru this with my 18yo who is not attending school FT and is working. We have the same rule - if you are not in school FT then you need to pay room and board. What we have decided to do is this - we are going to take all the expenses that apply to her i.e. mortgage, phone, heat, hydro, cable, internet and food and divide those monthly expenses into 5 because there are 5 of us living here. She will have to pay her 1/5 of the cost. For that she gets meals, a computer with internet, a phone to use, lovely home with her own room, her own bathroom, and all of her toiletries. What more could she ask for! Now, we will still expect her to comply to all the existing house rules as well as the chores she is responsible for because if she were living on her own she would still have to do things like laundry, cleaning, etc."
Read more about this thread here: Advice On: Adult Children - Teen's Room & Board