I am laughing out loud at these people saying it's too much.
My parents rules were 20% of ALL earnings were to be paid the day after pay day. This was applied to all the children. It had a cap of 200£ to start which was increased to 250£ over the years.
The rule was applied to anyone staying long term too (longer than a weekend) so if my partner was staying in my room he had to pay 20% of his earnings in that time.
A general rule like this keeps it simple and fair.
I suffered no finical damage because of this, in fact it taught me the value of money and prepared me for having my own bills etc.
I paid this from my first job over the age of 16 (ie paper rounds etc didn't count when under that age) until I moved out in my twenties. I'm happy to say I moved out with a true expectation of what my money would get me, and have never struggled with budgeting/bills etc. I truly believe this is because I was taught how to manage my money expectations.
I have friends who paid no "digs" (rent) and had a horrific reality check when they moved out. Many misjudged their budget and had to move home/run up debts etc.
In terms of OP relying on this money... So she should, she is heating the place, paying electricity, TV, rent/mortgage etc which will all be used by her son.
When you become a adult you should be expected to contribute a fair and equal share. If not... Go stand on your own two feet.