Should make 15.1 (13.1 plus two hands) but it depends on so many factors, most of all the height of bum related to withers, as long as he is growing he will be croup high. also if he looks "on the leggy side" he is more likely to keep growing, if you fed him mare and youngstock in his first year, and gave him plenty of running about he will tend to mature a bit earlier, but not always. Also depends on how much he takes after sire or dam, also if he had any setbacks when he was younger, this can stunt growth dramatically.