In terms of the Children's Act, maintenance by one parent to the other ceases when the child becomes an adult at the age of 18.
This does not mean that the newly turned adult may not require maintenance because they are studying, unemployed or otherwise in need of monetary help. It just means that it is now up to the kid /adult to approach the maintenance court to claim money from his parents, and his mom or father can't do it on his or her behalf against the other parent.
So even if the kid is still living with his mother, he needs to sue the father for his share of maintenance and then pay over to the mother his rent/board and lodgings.