The Law states that you, as the biological parent of the child and provided he has not been adopted by the mother's current husband, provide maintenance for the child until such time as he can financially look after himself and becomes self sufficient. There is, unfortunately, no set age within which this obligation ends.
It is worth it to note at this stage that this obligation rests on both parents, proportional to their means. What this basically means in practice is that the party with the biggest salary will pay the most maintenance, but the obligation rests on both parents.
This obligation is on you whether you have a relationship with him or not and this would include his tuition fees. If, however, you can show to the Maintenance Court that you cannot afford these fees, not for lack of trying, they will not order you to pay the Maintenance fees.
With your child turning 18, the situation also changed a bit, in that he now have to represent himself in these maintenance matters and that his mother cannot do it for him anymore. So, if there is no maintenance order that currently orders you to pay for the tuition fees, you are not obliged to pay for the tuition fees, but you will have to continue to pay the amount of maintenance until a maintenance court has set the order aside.
You can apply for the setting aside of this Maintenance Order in the Maintenance Court where the order has been issued. You should ask the Maintenance Officer to assist you in getting this application started.
Feel free to ask any follow up questions.