My friend had a similar issue with her just gone 21 son feeling hard done by for being "forced to hand over" his words- £50 a week from his similar salary, her ds was handing her a shopping list of the foods he wanted in eg fancy biscuits, more crisps, branded soft drinks as mixers rather than supermarket brands, as he was handing over so much money- his words. What's worse was he was working in a restaurant and could have free meals at lunchtime if he went to work half an hour early, but chose not to. So she just sat him down one evening, they went through the whole years gas and electricity bills divided by 12 and pointed out of the two of them living in house he used half, council tax for two rather than one, the sky package for tv and broadband,- she said shed pay half even though it was far more him than her, all household bills including the contents insurance , her car she paid for and he got lifts for free she brought him to work three days a week so she added in the three days bus fare, anyway at end they totted it up, she did it in a spread sheet - they agreed a budget for his share of loo paper, tea, coffee, sugar, washing detergent, toothpaste, milk, based on supermarket receipts etc which were all things he used, as anyone normal would expect- there was nothing left of the £50 for other food or drink. In fact he owed her, and then she reminded him he should be paying her rent not just his share of bills and shopping.
She said she didn't think it would make much difference, she was just annoyed at constant grief and chasing him for money, but she says every payday now he buys a few treats and has told her to help herself to his fancy biscuits, he gave her an extra at Christmas from his bonus and he's also been saving for a holiday with a friend and she heard him telling his friend that they needed a budget for food, drink sundries as they can cost a fair packet. Maybe something similar with your son would work when they see it on paper it becomes real