To get a rough idea if your pony will grow any more, all you need is a long bamboo pole or similar, stand it next to the foreleg. With a marker mark the position of the fetlock joint and elbow on the pole. Then move the pole upwards so the mark you made for the fetlock is in line with the elbow. If the mark you made at the elbow is level with the withers then he will not grow much more, if not he will grow up to the elbow mark - if you see what I mean.