Hello Leverage, the actual amount probably doesn't matter as much as the principle that nothing is free. By charging board you're teaching your child valuable principles and helping him to learn to budget.
I paid board to my parents as soon as I started work. I had no idea at the time that this amount was being put aside for help with our first home deposit when I got married.
My parents didn't need the money but they did need to teach me the value of money and the reality that we all have to pay our way. This sets up the basis, even at a young age, for a child who will grow up with the mentality of saving for his/her own retirement, and whowill eschew the entitlement view that their fellow taxpayer owes them a living.
Good on you for giving it proper consideration. All the best
