Look at how big his parents were and make a rough guess at the middle of the two. But some never make the height of either parent, whilst others are taller than both.
String test. Get a piece of string and measure from the point of fetlock to the point of elbow and then keeping the string in place at the point of elbow take the end at the fetlock straight up to the withers and that should give the eventual height. Tried this on adult horses on my yard (the two lengths should be equal in an adult) and all the different types including cobs, tb's and a rather tall warmblood were all the right height apart from my little connie cross who should be 16hh for her length of leg. She is actually just below 15hh! She covers the ground well due to long legs!