My horse is about 13hh & is a year old. How big will he be when he is finished growing?

He is fed on special Stud&Youngstock Coursemix, Molichop and Build Up once every two days. He is by a 15hh stallion out of a 15.3hh mare, yet he seems small. He is well built and healthy, lives out all year and shares 2 large fields with a mare and another gelding. In advance, thanks for all help!