Humm, i think In some specific situations, is the son married and living with his mother? If so, the mother has the right to discuss and collect rent, which is completely normal. The second situation is that the son is not married and lives with his mother. This is an internal matter between them and the agreement is to contribute to the living expenses, electricity, internet, water, etc. for the mother instead of having to pay separately. As long as they find a suitable solution, all parents love their children very much.
If I were the parent, I wont take house rent from my son or daughter, I will see it as my responsibility to shoulder but sill it will depend on my financial muscles at the time. If I'm strong and well to do, I won't put it on my ward to share the house rent with me instead I will encourage my ward to invest the money he's making into something profitable for himself and the family .